Roads don't change that often despite what the pesterings from Tom Tom claim. When they do, new roads often have good sign posts.
Yes, you could get into trouble like meathman did, but to be honest I occasionally got in that trouble when my Sat Nav was new and had up to date maps.
Oh and by the way I got into that trouble a LOT more before having a Sat Nav so not dissing Sat Navs here. I just think that in this "gotta get the latest thing" world we live in, you do need to remember that it's not always necessary.
In a few years you're probably want to upgrade your Sat Nav anyway and, in my opinion, it is not worth paying for the am updates in between.
Ultimately you're right SparkRite we do need more info before we can accurately answer.