I'd put it like this:
1) /NIN - Soloing, any time when you expect to take hate and not want to get hit
2) /WAR - DDing, where getting hit is okay
3) /SAM - DDing under very high Attack conditions (Red Curry Buns and Stalwart's always, etc) or main healing when someone else in your party has Haste.
4) /WHM - Potentially a main healing sub, if your group expects you to Haste them.
5) /RDM - This will replace /WHM at 99 sometimes

I have personally only used /WHM once ever. I haven't used /SAM since level 80. I went /WAR maybe twice in the last two months. Our subjobs give such minor benefits that shadows are very often more important.