Jimmy,
The stress is on the wheel bearings, so I don't know why it would affect towing. If your truck is aligned properly (mine isn't :lol
it should have no real consequences. I had to replace my bearings recently, but that's do to the camber, spacers, way I drive, etc.
Some shops will not do any kind of work involved with wheels/tires if you have spacers. You can always just take them off before you go in for service.
I'd recommend having a torque wrench, yes. I step mine up very methodically, and it takes a long time. 50, 75, 95. Test drive, take the wheels off, check the torque on the spacers to 95 again, put the wheels back on, 50, 75, 95, test drive, check the torque on the wheels. If everything is good, you're all set. I keep a torque wrench in my tool box in the bed at all times.
The lugs on the spacers should come out as far as stock ones do, and the spacers are designed to fit onto factory hubs, so they won't be too shallow either.
Here's a pic of my rears.
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