We often get questions from people who are getting conflicting advice from installers. One wants to install the panels flat, one does not.
My personal opinion is this: Do not lay panels flat unless you are prepared to clean them regularly. Flat panels can't fully self-clean in the rain, and grime build-up can be an issue.
If you do lay your panels flat - I strongly recommend panels with slots/channels engineered into the frames to help rainwater drain off. I used to recommend frameless panels - but I've since changed my mind. Why? Because in practice frameless panels require regular 'torque checks' on the bolts that hold them down to prevent them coming loose!
Here's a great blog post that Ronald wrote exploring horizontal panels in great detail: