I've read that it's important to check your credit score in order to keep a handle on your credit health, as well as to protect yourself from identity theft or fraud (for instance, in this Lending Tree article), but I've also read that that having too many inquiries can negatively-affect your score (see this post at Sweating The Big Stuff)
How much checking is too much?
My knowledge is mostly of US laws and rules, so I can't say for sure what applies to Canada
You do not usually check your credit score, you usually check your credit report. Your score is the calculated number that is used to represent you as a single value to a lender, while your report is a list of all the accounts and your standings with them.
In the US, you are untitled to a single credit report from each of the three major bureaus once a year. https://www.annualcreditreport.com/ is the only site you should get your report from. Some people choose to get a report every three months (once from each bureau) while others choose to get all three at once.
If you have a major purchase or suspect identity theft, definitely get a report and make sure you start clearing it up by writing letters and filing with the forms each of the credit bureaus provides for just that purpose.
As for checking your score, I don't really see the need for it. It is an interesting thing, once you know your report is clean and accurate, you can pretty much guess your number within 50 points.
If you are shopping for a major item and plan to finance (like a car or a house) try to consolidate all your credit applications around the same couple of week period to prevent the hard lookups from affecting your score. Hard lookups are when you are asking for credit, soft lookups are everything else.
Also, unless you KNOW you have been a victim of identity theft, don't bother with a credit monitoring service. Once you know a baseline that your report is clean, just keep an eye on all your accounts and you should know what your report says.
Also, those credit protection and identity theft services that cost $10/month don't do anything that you can't do yourself. They write letters to get your off the pre-approved lists, they request a lock on your credit report from the bureaus, and put your phone on the do not call list. All stuff you have the right to do on your own.
I think you misunderstood the post. The part about "soft inquires" says:
"“Soft” inquiries, ones that are made by creditors who send “pre-approved” credit card offers, ones made by you through an online service, and by potential employers, are not counted against you. When you request your own credit report, all inquiries appear. However, when lenders and creditors look at your creit report, only the hard inquiries are shown."
This is an old question and @McChrister has a nicely comprehensive answer. Still, I want to add a couple pieces. Step one is definitely to check your credit report for accuracy. http://annualcreditreport.com/ is the site to use in the US. You can get one report from each of the three credit bureaus once per year. So 3 at once, or 3 spread out over the course of the year. It's important to use that site though, there are many knockoffs with the word 'free' in them. Don't use them. As far as any other service, it's worth checking their validity at BBB.org.
Step 2 is the credit score. Again I agree with McChrister that it's not super important to check. Once you know it you'll be able to tell if it's going to change as you scan your free report every year. Still, it's not bad to glance at it now and again. Besides, you may need it for other reasons. Landlords will want to see it for example. And they want the score, not the verbose pages of report. They only care about the number. (Besides, it's impressive if you arrive on-site with your score in hand.)
When you query your own credit, it's a soft inquiry. Soft inquiries do not impact your credit rating and are not seen by others checking up on you.
A few more things...there are different credit scores though generally they'll be inline with each other. Each bureau calculates their own score. The easiest and cheapest way to get these bureau scores is through that annual report site. As part of the 'checkout' process, you will be offered a one-time option to purchase your bureau score. That sounds like an annoying up-sell, but the $7.95 is the cheapest you'll find. (I don't consider signing up for a free-trial monitoring service, giving them all your personal information and remembering to cancel convenient.)
The standard/benchmark credit score comes from the Fair Isaac Corporation. It's the FICO score and the one people usually want. Get it at MyFICO.com for around $20.
The only decent place I've found to get a free credit score is https://www.CreditKarma.com/. They have a very Mint-like approach - free service brought to you by advertising and up-sell recommendations. Their numbers are provided by TransUnion. They offer the TransRisk 'traditional' score along with a newer Vantage Score and an auto insurance score. Using the service does not generate an inquiry until you specifically mash a big get/update score button. Even then, it's a soft inquiry. Credit Karma is/has been a nice way to check my score even few months.
