Reasons why cupping the mic will result in a less optimal sound:
- You'll never have your hand at the very exact same place(s) on the mic, resulting in different sounding recordings. Which inevitable means that some parts of the record will sound at random better, some worse.
- If the mic isn't cupped, the engineer will be able to concentrate on giving you the most killer sound, without loosing endless hours hunting for resonances and mid bumps that change all the time.
- If you like the sound of the cupped mic that much, it's better to send the engineer a sample clip, so they will know how you want it. Then the album should be tracked without the mic being cupped.
Tips: Put the mic onto a stand. Experiment with the angle you sing into it. Use a pop-filter.