It does seem to be important how the re-hydration is done. It's best if you stick to the manufactures directions.
The temperature doesn't effect the viability as much as I though it would. In a test at four different temperatures from about 70°F to 90°F the viability was about the same across the samples. Because the bowls used to rehydrate the yeast were not sufficiently preheated the temperature dropped rather quickly back to ambient temperature. For the warmest test case the small ramekin was moved to a larger bowl of hot water to bring the temperature back up. The average temperature was calculated by integrating the temperature over time.
In the picture you can see there there is a fairly high density of yeast floating on the water. This proved to be a problem. As the yeast absorbs the water it gets a little heavier. At some point it reaches a tipping point where it sinks beneath the surface of the water.
Here's the interesting part.
The lowest viability was seen in the yeast that sank first, and the highest viability was in the container that sunk last.
From these results it seems that using a container with a large surface area is important for hydrating yeast. Sprinkling it on top of the wort in an ale pale looks like it could be a better option that using a glass. If the 150 billion cells that are typically in a package isn't enough for your beer you'll need to propagate them first. For propagating, hydrating the yeast in a shallow container (such as a large bowl or plastic food storage container) should work well.
Viability vs time the yeast floated
vertical axis is the viability as a percentage of live cells
horizontal is the time in minutes that the yeast floated
Four different test cases with different starting temperatures.
vertical axis is the temperature in °F
horizontal axis is the time in minutes
Viability vs Temperature
vertical axis is viability in percent live cells
horizontal axis is the average temperature in °F