it was true until ICC you mean. i remember raid healing in ICC on my priest as disc by casting nothing but PW:S and penance"Disc healing for tanks, holy for raid" is old school. That was true up until Cata. As 50% of this thread says, both are perfectly viable at whichever.
Holy priest have the same secondary stats no matter what they are doing (maybe crit being a little more valueable for tanking, but not by much), basically they just change their Chakra to dictate what role they are doing. Holy priests really, really depend on a lot of spirit, which is why a lot of people prefer disc over holy in FL, since there is not a whole lot of spirit gear in FL compared to caster gear that has no spirit and just secondary stats. Holy priests are a lot less mana effecient, but if you have spirit and know what heal to use when, you can make up for it and do a great job in either role.
Disc priests only require about 2200-2400 spirit to remain effecient, mostly because of the Rapture, which gives mana back on intellect, which means int (and essentially ilvl) is most important. You can get 2200 spirit in 253 gear and be an effecient healer, unlike Holy. Disc is good at either role, and anyone who tells you differently either hasn't played Disc in Cata, or is very terrible at it. Your secondary stats change though, and I'll share what those are below this. If you want to be be able to switch to either tank or raid healing, keep them balanced. You won't be gimmicked in 10man, and is actually probably recommended. In 25s tough you'll want to figure out your role and base it on that:
Mastary - Tank stat. Makes your PW:S & DA absorb more damage. Still beneficial for raid AoE mitigation.
Haste - Raid stat. Makes you able to spam PoH faster = more AoE heals, more DA procs. Can still be beneficial for tank healing, as faster GH = more reliable, and more inner focus casts.
Crit - Definitly a tanking stat. Compared to Mastary & Haste, less beneficial in both cases.