No, you can still use it. Just make sure that when you do the initial chrono that it's under the limits.
Reason it's recommended is because the hop ups are usually tuned for those weights. Most TM springer pistols with fixed hop ups are tuned for .25g BB's.
In CQB you will chrono with .20g then use whatever you want. Typically I rock .25's for FIBUA/CQC games (eg. in a "CQB village" or indoor field) and .28's for outdoor longer range games.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE