Just putting your monthly over-payment in a savings account generates a small amount of money, you still lose at today's interest, just not as much. A refund means that you are not managing your money well. What you get back, factoring in inflation, is less than you paid in. So it is nothing like a "small" savings account at all. By over-paying you are costing your family money. Intentionally losing money causes me to lose sleep. It's akin to telling the tobacconist to "keep the change" every time you make a purchase.
I'm not a miser by any stretch of the imagination. I burn up a certain amount of money every month through my pipe. But, giving the government a loan and allowing them to give you back less than you loaned is not a fiscally healthy way to manage your money. On the other hand, if you have more money than you need or want, I suppose it really doesn't matter.