This is a hard question to answer due to each server running different rules, some of which may be specific to their own server. The best advice I could give you woule be to use your common sense, if it is a race server don't drift around for example.
About drifting on race servers. I would say no don't do it, it won't be quicker than taking a racing 'grip' line. However, I find in the higher power RWD cars (especially LX6) at some tracks I end up crossed up at some corners nearly every lap. While it isn't the quickest way round, it works for me as I would have to driver slower to 'grip' the whole corner.
Edit: I would add, in my case I mentioned above, it is not really drifting as I am not trying to be sideways with large angle etc. Unless I make a mistake, the oversteer is corrected with very little opposite lock (1/2 a turn of wheel or less)