My experience with DK+4xRets is that I have been able to do just about any heroic I wanted (or needed).

I'll echo a few others...

I tend to run a random heroic each night for the frost badges, but I'll "turn down" queues for the Pit of Saron and Halls of Reflection as well as Oculus. Oculus isn't hard, I just don't like it and I don't have a character that can be dedicated healer for the "easy mode" strategy mentioned elsewhere. PoSaron isn't impossible, but the last section and boss of it aren't a sure-fire win for me and I'd rather just hit an easier heroic in the interest of time/efficiency. ToC is also often skipped, although I can't remember the last time it queued me for it.

On my gear-up path, I was able to do plenty of heroics even appropriately (or under) geared without an issue, enough to easily farm Triumph badges and get iLvl 200-232 upgrades. I made sure to do normal and heroic ICC bosses at the beginning of the instances for the juicy upgrades there, especially in PoSaron.

Unless Cataclysm is filled with more "challenging" 5-mans from top-to-bottom, I plan on using instance running as a leveling and gear up strategy yet again. I think I agree with others that the all melee team is nice from a no-downtime perspective and is faster than my shaman team was. If only they gave characters a point-blank, area-of-effect loot corpses spell.... mmm...