Yeast maintains it's viability very well outside of the refrigerator. Even after three days outside of the refrigerator viability will not change more than 3%. The results from a viability test of yeast even shipped across the country should be fairly accurate.
However, there is much more to yeast health than viability.
When considering yeast health the first thing that comes to mind is viability: the percentage of live cells of the total cell population. However, there are other factors when considering yeast stored at in the food danger zone. In this range of temperatures from 40 degrees F to 140 degrees F bacteria thrive.
Yeast that has been packaged for storage in a refrigerator is typically not stored with any food for the yeast because it is intended to stay dormant. When the yeast warms up to room temperature it will become active, and during this time the cell population will increase slightly, however it quickly runs out of energy reserves. The bacteria, on the other hand, will feed on the dead yeast cells, and continue to propagate even after the yeast have stopped growing.
During a test of two strains of yeast, WLP004 and WLP566 It was noted that within the first 12 hours both strains grew in population. Initial inspection of the yeast when it was removed for the refrigerator showed no signs of bacteriological activity, but after only 12 hours pairs of bacteria were observed indicating cell growth.
When the yeast is pitched into an environment that it thrives in, such as a typical wort, they can out preform the bacteria and the defects produced by the bacteria may go unnoticed. If the bacteria are given a head start, such as a day or two at ambient conditions, they will dominate the flavor profile.
In conclusion, I wouldn't pitch a wort with yeast that has been left out for an extended period of time, but the viability count and cell count should be very accurate even if the sample has been warm for several days.