Electric seats has nothing to do with it. I've never had 'leccy seats and on my 2 1'ers and my E89 I got dipping mirrors. It might be part of a pack (comfort?) but definately nothing to do with the seats.
I can see how the OP could think it was intermittent, cos if you dont make the link to the mirror switch position, it could appear that way, and its one of those things that once you know it, you just know it!
Would be nice if both sides dipped.
However, in the OPs instance, it would've been nice if the dealer had suggested that on the phone, a quick simple troubleshooting thing etc.
I took a car in once for tramlining and the first thing they asked was if the tyre pressures were correct. Duh, no, I hadnt checked them for months - they had great pleasure informing me they were all low!