First off, the basics on HST/GST:
You don't need to collect HST, if you don't want to, until you hit 30k in a particular three month period (assuming you're not regularly passing $30k). You then need to collect on the sale that takes you over $30k plus all sales after that. See the H&R Block page on GST/HST for example:
[B]usiness goes through the roof, generating more than $30,000 in one particular three-month period. In this case, the day the sale goes through that took you over that $30,000 threshold becomes the day you cease to be a small supplier. You must charge GST/HST on the sale that put you over the $30,000 limit, and on all sales after that, even if you are not yet registered. You now have 29 days to register with the government.
Alternately, if you hit 30k over four three-month periods (i.e., a year), then you are exempt until the end of that fourth three-month period, after which you must register and collect HST the month after:
[R]evenues in excess of $30,000 during four (or fewer) previous, consecutive three-month periods. You will be considered to be a small supplier for those four calendar three-month periods, plus the next month. Your first sale after that additional month, and all sales thereafter, will have to include GST/HST. You will have 29 days from the first day of the second month to register.
However, many businesses do collect HST/GST even under that limit, in particular as it means you can collect tax refunds for your input HST/GST paid. If you do so, then you simply register from the start, and then you don't need to worry about it. You do need to remit those taxes collected, though. If you don't remit, you won't be able to collect tax rebates for your input HST/GST.
You decide to become a GST/HST registrant when you start your business. You expect to exceed the $30,000 threshold at some time in the near future. You also want to receive any GST/HST paid back from the government on all expenditures especially those high startup costs.
And, as Grant Thornton recommends:
In most cases, it’s generally a good idea to register for GST/HST as soon as your business is established. Provided that your business makes (or will make) taxable or zero-rated supplies, early registration ensures that GST/HST paid on costs incurred is recoverable since tax paid prior to registration is generally not recoverable except on the purchase of inventory, capital property and prepaid services still on hand at the time of registration. Be sure to register early because, in many situations, registering late can result in the loss of recoverable GST paid before registration.