What plastic are you using when you have the heated bed turned on?
You don't need it for standard PLA. If you're using some form of enhanced PLA, where the manufacturer recommends a heated bed, then you should turn it on - otherwise don't bother, it'll cause more problems than it's worth.
If you're using ABS, yes, you will need the heated bed, but then it's unlikely you would have problems with the raft.
The problem is, that when I printed a wide lid for a jar the base of the lid warped upwards. I hoped to get that fixed when the bed is heated. How can I prevent the big round base of the lid to warp during print?