I used to do that and that worked really great until I bumped it by accident and the entire tray went on the rug! I spent the next day sorting things out.
Now I use these:
The lower ones 50 to a package. I mark them w/ blue tape. Often if it's part like the fans, or the optical drive I'll tape the screws into/near the holes where they belong. I did this a lot especially w/ the bottom screws from MBPs until I'd done so many I knew exactly where the longer ones went.