I would like to ask experienced stringers this topic. It seems like the reasonable thing to do is to increase the tension on the last cross or main, but I'm afraid that might damage or even break the racquet at the edge. Any suggestions?
Add about 10% (i.e. 2-3 lbs) tension on the last piece is the limit to me. Going beyond that is way too risky. With 10% extra tension, proper clamping method (i.e. use 2 flying clamps on the last 3 pieces) can very much minimize the tension lost.
And remember to pull the tie off main and cross strings with the following technique as illustrated in post #42: http://www.badmintoncentral.com/forums/showthread.php?t=43438&page=3 If you are using drop weight, let the weight sit for 1 minute or more.
Another alternative: string top down so the cross tie-off is on the bottom. The tie-off on the bottom doesn't have to be perfect, it doesn't have to be that tight because you don't hit (or at least you shouldn't) with the bottom of the racket. Too much stress on top is no good. If I string at 28lbs. or lower, I do a one-piece string job. That means the cross tie-off will have to be on top. Not a big deal. I recently experimented stringing at 30x33lbs. one-piece vs. two-piece and found the two-piece, top down, tieing the cross on the bottom was significantly easier on the racket than the one-piece, tie-off on top.
I'm using a Klippermate 140, a drop weight machine. If I let the weight sit for 1 minute at the last main or cross before tying it off, do I need to clamp it or without clamping it?
You still need to clamp it. Just imagine you release the tensioner and proceed to tying the knot(s) without clamping! Please look at posting #125 & #126: http://www.badmintoncentral.com/forums/showthread.php?t=22331&page=7