Whether the instructions call for pitching the yeast dry or hydrating it first, in most cases you can always hydrate it, anyway.
If you pitch the yeast dry, you won't know for up to several days if the yeast were viable or not. It is a guessing game until you see/measure active fermentation. That can be nerve racking.
By hydrating it before hand and getting the yeast solution's temperature the same as that of the must, if it takes a couple days for fermentation to start, you at least know the yeast were viable when added, so you just wait.
An even better way is to create a yeast starter, where you hydrate as usual and slowly add your must into the solution to get the yeast well acclimated to your must before adding it to the whole container of must. You then gently stir the active, bubbling starter into your must, knowing full well that the yeast are alive and well.
Again, the temperature of the yeast solution should be within only a few degrees F. of the must, otherwise you can shock the yeast.
Most of the time, visually active fermentation starts much sooner with a yeast starter than when just simply pitching the yeast. However, from my own experience, I have found that some cold fermenting yeast, like D47 (59F to 68F) , still may not visually seem to take off for up to 24 hours after stirring in the yeast starter. Once it gets going, though, it really gets active for such cool temperatures.