Question: Last weekend I was re-assembling my engine, and three holes stripped, is there any way to see if a screw hole needs to be re-threaded before it strips?

Answer: 
If you want to eliminate the angle of ground that visually looks flat, measure the weight then turn the whole lot around and measure the weight again. The mean average fo these two readings should give you a fairly accurate idea of the true, level nose weight.
That's close enough! Especially if you're not pushing the limits.

Tongue weight influences the stability of the rig (too much and the front wheels may tend to lift, too little – especially with a heavy load – and the "tail starts to wag the dog") and the load on the towing vehicle's suspension. You've got some latitude there, probably more than the difference that a hard to see slope in your driveway would amount to.