I live in Wisconsin and will be having a lawn service company applying a spring fertilizer application the end of this month (March) or early April. Should the lawn be de-thatched first or does it matter?
It depends. If your surface is compacted, or has a thatch layer on the surface, aerating it or de-thatching it with a power rake is a good idea. You could ask your lawn service to do it first, or you could do it yourself by renting a tool. Power rakes are really overkill if you just want to aerate the lawn. There are also power tools that remove plugs every few inches. That allows rain and sprinkler water to penetrate to the roots.
The most important thing is to have it evaluated as to whether or not it's needed. If you had a decent lawn last year, it may not be necessary.