basicly its simple for locks .... for raids you spec affliction when you start raiding ... stay it in the whole T4 bracket ( Kara - Grul - Magtheridon )
once you got gear from the T5 bracket ( SSC and TK ) you can start thinking about respeccing Sacrifise / Destro if you can get at least 20% crit and 150 spellhit (the more spellhit the better) at this point stick mostly to spellhit since you get aggro problems anyway unless your tanks are realy awsome geared
in the T6 bracket 1 or 2 warlocks stay affliction the rest goes plain Sacrifise / Destro you should be hitcapped at that point anyway and got at least 25% crit ... thoose affliction locks are for raid suport ( make sure you take malediction ) and drain tanking at illidan mostly