It's not an issue and you can mulch any time you want. Decay, however, won't proceed at any great pace over the winter.

A light to modest leaf layer won't hurt the lawn either, so you're safe either way. I still have a pear tree unleaving out there myself and I'm currently ignoring it. I'll probably break down and pull out the mower just to make myself feel better.

It's generally considered unwise to mow the lawn at below-freezing temperatures--it damages more easily. So do the mowing at some point when temperatures are hitting forty or so.

It will depend on how dormant your grass is and whether or not the Maple leaves will flatten out of the ground and suffocate the turf. Maple isn't as bad as the Oak leaves from which I've ended up with many Oak Leaf shaped bare spots in the Spring. Little piles are even worse.
Crab grass may sometimes grow in those open spots before they fill back in. The Michigan study showed that mulch mowing Maple leaves hinder crabgrass germination so I would go ahead and mow for those reasons.
To me it is always the concern about long winters with leaves suffocating the grass plants, and WI definitely has long winters. :)

Thanks for the info.