The rule of thumb I've used is 1 amp/channel=no heatsink, 2 amps/channel=heatsink.
While you probably can get aways w/o putting a heatsink on for 2 amps, why not just do it to be safe. I would think that if someone is hitting 2+ amps / channel w/o a heatsink, then they might not be doing it on a high load (continuous) basis, ie hit that channel once every 20-30 seconds vs all on all the time (hope that makes sense).
You really need to look at the amperage of the string. I have 100 count, "energy efficient" strings that are .20/.24 amps vs .33-4. So I can get 4 / channel. Take a look at the string on the safety sticker and it should tell you what the amperage is.