This is why I carefully select my nobles to ensure mandates disrupt things as little as possible. First, I use the splintermind DT branch to evaluate dwarves for what they're good at and by the second autumn migration wave I go over the various dwarves who have been relegated to hauling to see which one will produce the least annoying mandates as a ruling noble. That dorf gets assigned as expedition leader/mayor as well as manager and bookkeeper and is recommended for the position of baron(ess) once it's offered.
My current duchess was chosen because of her relative uselessness and because she likes coins. Coins are a great preference for your ruler unless you have no metal at all for a few reasons. One, there's absolutely no quality level at all for coins so you can melt them all down without another dorf getting pissy that his work of art was destroyed. Two, I think the rate of bar return on a stack of coins is 1.1, so you actually end up getting a little extra back every time you melt them down. And three, coins are crap for trading so you're not going to have to worry about export mandates either.