For me, it depends a lot on what kit it is but I always look trough the instructions on new kits (especially new tool kits) to see how things are done and to get an idea of what details are in the kit. But I don't always use them when building, many times only for refrence on some parts.
When I build something I am familar with in 1:1 then the instructions are of no use (like the 63 bajabug project I got at the moment) as I build from what I know but if I build something that I don't know much about then the instructions are used all the time.