After putting Launcher Pro on my OG Droid a long time ago, I went gung-ho with 7 screens. After a little while, I reorganized me homescreen content and have been using 5 ever since. I feel like 5 is generally more convenient for me. Seven screens got to the point of being too much for me. I found that I almost never used the widgets/links on the two outer-most screens (which is where I put the least-used stuff, of course), so I decided I could do without them.
-SR-