har217, I think/hope you'll find it's a matter of technique, and to some degree, equipment. I use mainly the 3.0SC and 2.5DC replacement cartos on a device that is set to 4.2v (for the 3.0ohm SC) or 3.8v (for the 2.5ohm DC). Higher values than that tend to scorch the juice, as mentioned above.. If you are using it with a MOD, if it is not variable voltage, it may be fixed anywhere from 3.7v to 5v (possibly higher, but I haven't personally seen that). My PT and non-variable MODs are all 5v, which with the lower ohm cartos, definitely risks scorching, but the 3.0 ohm sc (similar to cool carts) works best on them for me.
If it is indeed scorching, and not filling technique, Mr Morandir posted a very informative post on the wattage levels that various juices could handle, which may help you in determining the right level for what you want to do.
I hope this helps more than confuses!
EDIT: Fixed reference above that said 3.0 ohm scorched for me, which was exactly backwards of what I meant!