Soft body dents are typically from ice or birds. Dents predominately on one side are usually traceable to leaking water drains.
Birds struck at speeds above 150 kts have been known to cause small dents for birds weighing a half kilo or more.
Dents with creases or gouges are typically your service to aft galleys, etc. Stuff off the gear typically won't get raised up that far (depending on A/C).
Seen lots, but never recall a problem.
If you want to save fuel that much then power wash off the bug chiten or hire some cheap 3rd world labour force to wipe down the bottom of your plane.