You can if you want. How many games do you need to have immediately available? I don't because it's one extra step.
it always depends. for instance if a game is set up on 128 Mbs you can trim it and take off 64 Mbs (well 63.99 etc) or you might just end up with 127.99... etc. saving next to nothing. When i first started to trim due to lack of memory i found a site stating the exact size after trimming and the next binary value(ie the one it starts at) with which i could see exactly how much memory i am saving before trimming. But alas the website is lost and google will not provide it. I suggest always trimming to save time and hassle to do alot of games and then finding out you still haven't room for your new game.
