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The most direct all-Interstate route from Seattle to Detroit is right around 2300 miles. To try to go "around the Rockies" by heading south to L.A. and then across through Arizona and New Mexico would raise that total to over 3500. That's roughly 50% farther, which means 50% more time on the road, which means more chance of an accident, greater cost, greater fatigue.... And no guarantee that you won't see winter weather. In particular both I-5 (e.g. Siskiyou Pass) and I-40 (e.g. over 7,300 feet in elevation in Arizona) see significant snowfalls as well as ice storms and sleet during the winter.

Since driving all those extra miles would require you to be on the road for an additional two and a half days doesn't really make your trip any less prone to bad weather, you'd be better off using that time as a buffer to use IF you actually run into bad weather on the more direct route, basically I-90. Even if you were to see a snowstorm while driving that route and stayed off the road for a full day to let the storm pass, the road crews do their job, and the sun to come back out, you'd still be a full day and a half ahead of taking the far more circuitous route "around the Rockies".

It is also worth noting that Interstate Highways are built to standards that severely limit how steep any grade can be (less than 6%) and how sharp any curves can be. If it weren't for the scenery, you probably wouldn't even know you were in the mountains. Remember, these roads were built to let the big rigs drive at constant speeds in the 65-70+ range. They are certainly not something to be feared. So, in short, just stick to I-90 and remain flexible.

AZBuck