I've seen a few irregular bet sizes on PS, but seldom make them, I generally just type in a rounded figure, to represent a portion of the pot, or a multiple of the big blind.
I've seen some people bet the exact amount a player has in front of them, especially in a cash game, whether this is a slight extra bit of intimidation I don't know, never done it myself, but I do see it now and then.
One more possible reason to throw into the mix is to try to cut down on rake paid in cash games. Any time the bet is sized so that the total pot size ends up just short of a rake interval the players save money. For example, if you are playing .10/.20 online and there is $2 in the pot, betting .90 instead of $1 will mean that the rake for each dollar is taken out only one more time instead of twice which would have happened if the bet were $1.
Thats a great idea, never thought of that before. Not something I have to worry about too much at 1/2 or 2/4 limits, but as the stakes increase so will the rake, I guess. Something worth looking into.