Maybe I'm to paranoid about strangers working on my 2012 RX 350, but when mounting new tires,
what's the best way to make sure some monkey doesn't scratch up the alloy rims.

I had a unpleasant experience with this before.

 

Unfortuntaely there isn't a way to prevent someone from mucking up your rims when you get new tires. It can happen at even the best places. All you CAN do is make certain the spotless condition of them is noted and mutually agreed upon on the service sheet prior to anyone touching them. Then thoroughly inspect them afterwards and if you see damage point it out as not being there prior to the change and ask that it be fixed at no cost to you. Inconvenience, yes, cost, no.

 

This is what scares me when getting new tires.

 

I had my tires changed last week by a local tire place I've dealt with for years. I've purchased non OE tires from them in the past and brought them to the dealer to install. This time I had them do everything and was very pleased with their work. Of course I did watch them like a hawk, LOL.