Almost every instructor I have ever had has recommended putting your thumb on the hammer(as to take it off the grip safety on 1911) when holstering the weapon. The reason for this is two-fold:
#1. If you make the stupid and dangerous mistake of letting your finger(or something else) slide into the trigger guard while holstering.
#2. If you make the stupid and dangerous mistake of not putting the slide safety on before holstering.
If your thumb is on the hammer & off the grip safety it helps insure a safe holstering, by not having a necessary safety disengaged.
Of course, this is only good for grip safety pistols like the 1911/XD. With DA/SA pistols like the SIG or Glock, you need to keep that finger STRAIGHT!!!!!
-Fuji
One more thing, IIRC, the XD has a slide-stop mech. built into the grip safety. If the grip safety is not depressed, the slide will not release. So, it is a good habit to holster without depressing the grip safety. I could be wrong on this, but I remember playing with one and not being able to move the slide until I depressed the grip safety.