Bet the same % of pot everytime, or dont bet at all avoids giving tells away. Go smaller in 3 and 4bet pots (or mulitway pots) because you can get the money in easier with small bets and your bluffs are cheaper while maintaining same efficancy.

Betting the same % on every board is a leak. On Some boards you dont need to bet like66%. For examplle k72r. There is a half pott bet more then enough

Depends on what stakes you're playing. If it's 50nl or lower you definitely don't need to worry about a small fluctuation in cbet size, nobody will ever notice it ;). Especially if you're playing zoom.
Ofcourse in the long term or at higher stakes it does matter and you should have different sizes on different boards and the same size on the same boards (you might even have 2 sizes on one board, but then they should both be balanced).

Something to remember for size: The drier the board the less you have to bet. The more draws there are on the board the bigger your betsize. This will be true in most cases.

I have a 33%, a 59%, a 74% and 150% pot. 33% I usually reserve for 4bet pots and some really dry boards. 150% I usually reserve for river spots. To decide between 59%/74% in a flop cbet type spot I pretty much just decide how polarized I want to cbet and use bigger if polar, smaller if more linear.