My understanding of the Edge 500 is that it doesn't replace cue sheets, because it doesn't show maps or tell you when to turn.

If I'm concerned (or curious) about a new ride I will check out the cue sheet on google maps before I leave. In general I only have problems following cue sheets when there is an error or a sign is missing.

I only use the map on my phone by stopping to check it if needed. While I'm riding the phone stays in a ziploc bag in my bento box.