Now don't get me wrong, I would be totally ok with this.. but I don't see it happening.
This is a very common tactic amongst a LOT of companies (Playstation, Steam, Xbox, most other companies where you buy some sort of credit through them). It brings in more money for them, since very few things actually cost $5 or $20. They cost $4.99 or $18.98. Just a little less than the credit amounts. A penny to the individual user doesn't really matter, but when a million users have that extra un-usable penny in their account, that means a pretty big amount to the company.
That being said, Gaia is no where as big as Nintendo or Sony, so I don't think that extra penny, or even an extra dollar, is going to be a HUGE difference in their profit, so it would be nice to allow the user to customize how much GC they want.