Although I haven't had an unneutered male (Max had already been done), my understanding was that it's best to neuter them as soon as their marbles drop, because this means their testicles are fully developed and producing testosterone, so it won't be long before they start exhibiting tomcat behaviour such as spaying, which could continue after neutering if they get into the habit. I think some people like them to develop that chunky look which male hormones give them.
I suppose this depends on whether there are unspayed ladies around, or women who don't fancy having their knees bonked (this happened to me with a neighbour's cat, and there was me innocently thinking his was 'making his bed' when there was a shudder from his bum, and a wet patch on my knee!).
I think after their wotsits have dropped is a logical time.
With females, maybe about 5 mths; earlier for orientals.