As mentioned earlier, hanging it by the front wheel is often GOOD for the fork as it keeps the oil in lowers up around the bushings.
As far as hydro brakes... it could be an issue if you have air in the lines, but if you just make sure not to cycle the brake lever (while it is upside down and for couple of minutes after you turn it back over), it will greatly reduce the chance of forcing it into the caliper. It probably depends on the model of brakes how big of a problem this is. With my XTs, getting a bubble out of the caliper can be a real b!tc#. But I’ve never had a problem on the few occasions I have stored the bike with the fork up. I have had issues with air in the rear line, but never the front.