You're fine. As long as you wait for the cooldown period before voting each time, you're fine. The only time you're ever breaking the rules is if you're using more than one forum account (at the very least; you may also need to change your IP, but I'm not sure about that - what is certain is that the only way to vote fraud is if you have multiple accounts). The cooldown time is designed so you can vote multiple times, that way anyone (even non-donators) can accumulate some ad credit over time. Vote fraud is where you try to vote more than once without waiting for the cooldown time, by creating a new forum account (where the cooldown time will be ignored). But as long as you wait for the cooldown, you can vote as many times as you like and that's not vote fraud at all.
Incidentally, when you say "daily" - you do know some of the sites can be voted for twice every 24 hours? To accumulate ad credits as quickly as possible (without breaking the rules), vote for each sponsor as often as you possibly can. For those of us who live a scheduled life, that means voting at a certain time every day. Totally worth it
