My spares are/were very limited.
In WI where power was potentially an issue, I had an RV with a 7500w diesel generator capable of running ~10 days or more on the fuel I had. In ATL, power is not an issue living downtown so I never worried about it.
I had backup heaters, and a few powerheads but that was it.
What I do have is the ability to spend money in an emergency. When our biocube 29 split many years ago, we got everything into buckets, picked up heaters at a 24x7 wal-mart and were on the doorstep of our LFS when the store opened. Dropped around $1500 that morning in the emergency upgrade.
I understand that many people don't have that luxury/ability, so picking up critical spares along the way is pretty important. For me, it just meant that I could, at any time, be hit with a sizable expense and I was willing to take that risk.