Honestly, this depends on the quality of your components. I can't say for sure, but there may be face wear on either of your RD pivots. This can cause the derailleur to move into a shift position, but move a bit, causing uncertain shifting. If it shifts when you're riding it, I'd say go with it. If it ain't broke, don't fix it.
Also, it may have something to do with the tension on the cable. Try increasing the tension on the adjuster barrel on the derailleur itself.
Last edited by DRietz; 12-14-09 at 11:27 PM.