I at least make sure it's doing something after rehydration before I pitch it, if it's not doing anything obvious I add some must as if I was making an acclimated starter...
I keep my yeast refrigerated so it probably extends their viability for at least a couple months but I do try to use everything before it expires.
I did underpitch once with a packet of yeast that was expired by a couple months and hadn't been refrigerated (the packet that came with a wine kit that took a while for me to get to, survived at least one summer in my kitchen) and it took a while to get going and wasn't as quick as most wine kit fermentations I've done but it did eventually finish where it was supposed to.
"The main ingredient needed is 'time' followed closely by 'patience'." - The Bishop 2013
"Good grief! If someone wanted to murder you, all they would have to do is ship you a 55 gallon barrel of honey and watch you document working yourself to death!" - Vance G
"When you consider that laziness and procrastination are the fundamentals of great mead, it is a miracle that the mazer cup happens." Medsen Fey, 2014