I usually go 24\0 with CFLs as it promotes faster growth and doesn't requier a timer.
When I use big lights for veg. I go 18\6 when the dark cicle is at the hottest time of the day. It saves energy and helps maintaining uniform temperature over the day.
CFLs doesn't give out enough heat for me to be concerned about it that much and I rarely go over 100W with CFLs (usually using a singe 45W) so energy consuption isn't that much of an issue as well.