In general, following the W-4 instructions should result in withholdings that are fairly close to the amount of taxes that you will owe for the year, particularly if your situation is relatively uncomplicated. Claiming less withholdings than the form suggests can help ensure that you end up saving money in your "interest-free IRS savings account" and get a refund at the end of the year, which some people prefer so they don't need to budget separately for a tax payment. I'm guessing that the HR employee either prefers doing so himself or has on occasion received complaints from other employees that they "didn't take enough out".
Personally, I'd prefer to claim as many withholdings as I can, and be sure to have some money aside in case it turns out that I have to owe a little bit, since it means I get more take-home pay throughout the year.
It's good to keep in mind that a W-4 isn't written in stone. If it turns out that too much or too little is being taken out, you can always change it. You can also try playing around with the IRS withholding calculator to try some scenarios.