Nope. What do you think happens if you were to fill the 5 gallons then drink 2 gallons. You'd have 3 gallons left wouldn't you? It's all good in the hood.
Just fill that keg with CO2, then rack the beer in, then make sure you purge it with CO2 again. The only fear is headspace with beer staling O2. Make sure you push out the O2 and you are golden.
This was actually EdWort's Apfelwein. Racked the beer/wine/cider (what do you call it?) to the keg. purged it 5 times b/c i was paranoid set it to 30 psi and put it in the fridge. Then I shook it once before bed and once in the morning. By 24 hours it was most def carbed. I set the pressure to 6 psi, purged the keg and it still foams a lot. I'll have to try some tonight after work to see what its like.