There is discussion of this in the penultimate chapter of my new book.
As others have said, it's a balance between running short of cash (and having to pay borrowing costs with a spread), and FX tracking error (large currency balances creating p&l over and above what you earn in futures). Personally I'm quite relaxed about the latter, which means my FX can add/subtract 3% a year to performance.
So for example right now, I have 10K CHF, 11K EUR, 85K JPY and about 150K USD in surplus cash (not used for margin). If I was less lazy, I'd sweep those balances to GBP every night, and sweep back if I needed margin. But I'm lazy. Also, this (daily currency funding by trading with central treasury) is one of the jobs I had to do as the desk junior in my first IB trading job, so maybe the activity triggers bad flashbacks.
(I guess I could write code that did this, but I'm too lazy even to do that)
And that would mean that the carry strategy wouldn't work in FX, but it does; the theory is nonsense.
Having said that, I wouldn't allocate cash in a futures account according to interest rate premia, as that's effectively doubling on the FX carry strategy that I already run in futures (and it's a poor way of doing it, because of the broker rate spreads that aren't present in futures).
Rob