How important are resv at campgrounds on Alaska Highway

It simply depends. Depends what time you get off the road. If you get off by mid afternoon, no problem driving in. Depends on weekday or weekend. Weekends the locals are out jut like anywhere else. Depends if there is a holiday weekend of fishing season start. Depnds if you are wanting to get into a high demand CG like Teklaneka in Denali NP. Working around all of that is not hard to wing it with no reservations.