Yes, it is. Or at least knowing the water activity can provide an estimate of shelf life (about the only indicator there is, aside from just keeping ganache for a while and checking it for spoilage from time to time). Jean-Pierre Wybauw gives the following guide for how long a ganache can be safely kept:
for an Aw greater than 0.85: maximum of 3 weeks
for an Aw between 0.70 and 0.85: maximum of 3 months
for an Aw between 0.65 and 0.70: maximum of 9 months
for an Aw below 0.65: "the ganache is microbially stable" (I'm assuming that basically means "forever")
I operate on the general principle that the Aw must always be below 0.85, with the goal of getting it as low as possible. In the U.S. some states have specific regulations that nothing at 0.85 or higher can legally be sold.
