When using routing, there are audible alerts in advance of turns so that could limit the amount of time you need to look at the screen. Also, in some display modes, you can have a fairly large direction arrow which is easy to see at a glance. Overall, I think careful planning of your route and waypoint names should minimise the issue, if it is an issue at all.
Some GPS devices have better screens than others in poor light conditions - overall, the Etrex HCx seems to be among the best for cycling purposes in this respect.
The HCx is very angle-sensitive so some experimentation with mounting it will pay off. You could also use a mount like a Minoura to bring the device up off the bars a bit and closer to your eyes.