There is no raid boss in the game which requires frost resistance gear. It can kind of help out the first few times you kill Sapphiron in Naxx10, but you generally gimp your DPS too much by wearing it, and really for casters its totally a waste since they have so much range and can move out of Blizzard quickly (the only really deadly frost attack he does). In naxx25 frost resist is a waste, its better just to burn him down fast, especially with more healers.

Some speculate Icecrown may have frost resist bosses but that really is months and months away, and I would guess by that point there is "better" frost resist gear.

For Novos it may be interesting to equip some frost resist gear and try it out, but the gear tends to be stam heavy and not so great on other stats, so I wonder if you would be gimping too much on mana pools and DPS. Be interesting to try anyways!

To be honest my strat with Novos is simple - do him once just for the achievement, then always skip him