Yes, it improves your reputation with the faction you sell it to, though not as much as it used to.
Absolutely not. If I'm out for money, I scan only things which are worthwhile.
Though I respect people who take the time to do a thorough job!
This basically comes down to:
I scan the system with ADS.
I scan the star I arrive at. (~1200 credits)
I scan any small planets which are within scanning range without moving. Generally they'll be High Metal Content (~ 2500 credits) or Metal Rich (~ 5500 credits).
I scan any large planets and stars which are within scanning range without moving.
If I spot anything which looks like a Water World (~ 13000), Earth-like World (~ 29000) or Ammonia World (ha ha, ~ 17000) I'll go scan it wherever it is.
If I spot a White Dwarf (~ 11000), Neutron Star (~ 19000) or Black Hole (~ 20000+) I'll go scan it wherever it is.
If I spot something which might be a High Metal Content / Candidate for Terraforming (~ 19000) I'll go scan it.
If I feel like it, I'll chase down extra Metal-Rich Worlds (~ 5500) and Class II Gas Giants (~ 5500).
I'm getting to the point where I can identify most good stuff right after firing the Space Horn.
In other news...
This is a 112 Earth-mass Water World. Mental. Yours to see at HR 5998.
And I got the values for some HMC / CFT planets as hinted above.