My experience with mechanics who have lots of stories about what does and doesn't cause rotors to warp while trying to sell you stuff is that they are exactly that: stories.

I've bought lots of rotors on line. No problem with that, unless of course you're buying unknown junk off ebay, etc. Zimmerman from a known seller is fine.

Bedding in is very important when you replace rotors. One thing you did *not* mention is whether you let them cool down during the process. The repeated faster stops heat the pads and deposit pad material into the face of the rotor (that's the point of bedding in). If you then stop and sit with your brake on the pedal, that will screw up all you just did.

Once properly bedded in, in normal driving you don't have to worry about sitting at the light with your foot on the brake. Now if you've been playing boy racer in the canyon for the last twenty miles and the rotors are nearly glowing, drive in light traffic for fifteen or twenty minutes and let them cool down.

On newer MB (since like late 80s?) the rear rotors seem more prone to warping than the front. Perhaps it's because they are relatively thin and not vented, who knows. They're cheap enough, just change them out.

My guess is your guy was shotgunning his guesstimate and tried to explain himself out of it once you told him you just replaced the fronts. That tells you he didn't really inspect things properly, just wanted to work up an estimate. Whether you want to trust him as a mechanic may be an entirely different issue; I tend to try and avoid business relationships with those who exhibit dishonesty. No one's perfect, to be sure, but this is a bit more weasel-like than I'm comfortable with.

Good luck.