I have noticed the need for re tightening my belt is due to it slipping. Not stretching. I think something better than electrical tape needs to be used.
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I haven’t noticed and belt slippage/stretching. I probably don’t use my machine as much as a lot of the people on here but no issues here. I didn’t put heat shrink on one end of the belt to aid with slippage (more preemptive than anything, also forgot to but it on the other).
I had mine stretch a bit after the first use, then settled in.
I am not sure “pre stretching” would be all that useful. It would be hard to get the right amount of force to duplicate it. And I am not sure that the belts would not rebound once they are left loose.
I think it would be better just to update the instructions to have people check and adjust their belts after the first few hours of carving.
Stretching is very unlikely as they are reinforced. Slip, sure. 
You know what the reinforcing material is? I only ask because I built my machine last weekend and had all three belts nice and snug but while waiting to some missing parts this week I noticed last night one was much looser than the others. No slippage as I used heat shrink tubing. Simple to tighten it back up again but they do lengthen a bit.
Fiberglass was noted previously.