What sort of a ridge? You should be looking at the RS of both pieces of the work as you graft. The "ridge", if any, would be on the inside. If you are doing the grafting with the WS facing you, your ridge would end up on the RS of the work, which is not where you want it.
If you're working the graft on the RS and you still get a ridge, you may be pulling the stitches too tight and it's puckering. I've found that you really have to do a good number of graft stitches before it starts to settle down and work its way out.