I think there has to be a good, solid reason to geld them.
If they're well behaved, not getting too heavy and they're easy keepers then I can't see the point in gelding them for the sake of doing so.
On the other hand, if they're bully bastards and pains in the arse around the stables or if they're dangerous or if looking like they'll be a bit thick set and heavy over their front end then that's another story.
We gelded one of ours a few years back now simply because he was looking like getting so heavy around the neck/shoulders area but he already had front feet problems so it was a bit of a preventative measure because otherwise he was a real gentleman.
Also, some colts simply get such a big swinging set on them that they'll pinch themselves in the score up or otherwise during the early stages of a race and often go off-stride as a result. People will geld them thinking that's some sort of a temperament thing when it's just a pain thing. They're smashing their own Jatz Crackers. Either way they more often than not have to come out anyway.