Lon, it has been brought up, many times (but remember, I have been on this site for a billion years).
No, it will not affect value. I kinda believe if your gun is a bit worn and you re-checker them and make them look new, then yes, folks will do a double take.
But I have cleaned numerous grips and it just makes them look nicer.
Personally, I use several things, but my favorite is either Simple Green or Murphy's Oil Soap, either work excellent and then after they dry, you use boiled linseed oil or the like on them and they look good.
Be advised that after you clean and they dry, they will turn grayish and look awful
Ed