Sprinkle or rehydrate? We strongly recommend simply sprinkling dry yeast on top of your must. In order to rehydrate your yeast properly, you need to have precisely 10 to 1 water / yeast. If not enough water is used, the yeast will grow sluggishly. If too much water is used the yeast cells will burst.
Temperature range is inflexible. If the water isn’t warm enough, the yeast cells won’t soften. If too warm, the yeast cells will cook and die.
Then, before adding your rehydrated yeast to the primary, the temperature difference between the yeast and your wine or beer kit must be less than 2 degrees Celsius. Otherwise, weakened cells may die and others may go dormant.
Seems like a lot of trouble, doesn’t it? Just trust the yeast, it knows its job. A short lag time isn’t enough to allow any spoilage. It’s a lot simpler, and why take all that risk?