If it was possible to do well in a slower light mech i would lower the engine a bit for more firepower, however slow lights are dead lights so i max the hell out of them.
I tend to run max engines in mediums unless the engine cap is outrageous, like the Cent-D or the trenchbucket with the max 390. (too much weight for too little a speed increase, plus the lack of firepower is rather unsettling for me)
Heavy mechs i tend to run around their default engine speed unless it happens to be too slow for it's weightclass/my tastes. (also wtf pgi, you took 5 kph off the default top speed of the QD and the dragon, did you really think that extra 5kph was that OP? , whatever, it was probably to compensate for something like speed tweak or orbital wobble or something else just as stupid)
Assaults are kinda random for me, depends on the individual strengths and weaknesses of each chassis at that point. (i might run an atlas with a max engine because i have the tonnage, but i'd reduce the engine on a highlander because i deem the extra speed not as useful as more weapons/cooling/defensive tech.
TL;DR
I pretty much try to run the max engine on every mech unless the max engine rating cuts too far into firepower/defensive quality or if the gain in speed from a higher engine is negligible in contrast to the possible gain in armament.
Edited by Slashmckill, 19 June 2013 - 07:32 PM.