Im sure it is fine. But with time, yeast viability is going to be greatly reduced, and several step ups sounds time and labor heavy. At least compared with getting a new smack pack with 90% viable yeast. I more cautious then most brewers though, I keep my yeast fresh as possible and never go past 6 generations. 3 months just sounds like a long time for a starter, I would feel different if it were properly stored with glycogen and yeast nutrient.