I first started buying in 2012 and have seen quite a few decent jumps and drops. I'd see SOME dealers increase premiums for a short period but they'd come down. That's just how it's seemed to me from what I have seen.
Also, on a side note, I don't believe dealers can look at premium as a percentage over spot, I think they'd have to look at it as a dollar figure over spot to make their business work(again from 'my perspective') so that'd create a percentage fluctuation of the premium (ie $1 is 10% of $10 but 5% of $20).
As for promotion using part of a premium, it shouldn't be affected by a change in spot for a properly hedged dealer. Am I right here?? Not sure