i actually think it is good practice. if there is any issue with the gig, they eg try to force you to hand over access to the account, or someone is breaking into your github account then you don't expose all your other work.

there may be an issue with github TOS not allowing more than one free account per person though. i don't know if that is still the case, but then i would get a paid account for each paid gig and just add it to the bill.