I believe it is fine and OK to tip a person for going above and beyond the call of duty, no matter the type of job (unless tipping may be considered bribing of some sort and become illegal). What I don't believe in is so many people tipping, even if the service is just standard. America has way too many services these days that 'require' tipping. A delivery person, or any other person for that matter, should be able to work and do their job average, and get to keep their job and earn the income the company gives them. An employee that goes above and beyond what they are required to do may receive a tip for their extra effort, but should never expect it, I think.
I am all for tipping a cleaning lady for doing their job exceptionally. On the contrary though, I would hate for it to ever become 'accustom' to do so. If someone actually DOES do a terrific job, it deserves reward. If they are doing average to poor though, they should never expect a tip. I wish all things were this way.