As long as your drill holes are perfectly straight then they will lay flat.
As far as placing them under components, ive never had to, but i guess it would depend on how low to the surface your component lays and the thickness of the insert.
Also take into consideration the thickness of solder you apply to ensure a solid connection.
Using a desoldering braid after applying solder should cut down the thickness.
I dont remember what company i bought them from, and i dont feel like looking through all my catalogs, but if you want to search for them they go by alot of different names.
I did a quick search and found a few links for you guys.