Yes and no.
I have ammo in every car that my wife and I own and drive. That said its not so much a stash as it is a convenient supply of ammo for range trips, hunting etc. I don't just have defensive ammo in that supply (actually I don't have ANY defensive ammo) but rather I have a few hundred rounds of each type of round I think I might use on a range trip, hunting, camping etc.
As such I have a locked tupperware tote with over 1000 rounds of .22lr, 500 rounds of 12ga. #6 bird shot, 200 rounds of 12ga. 00 buck, 300 rounds of .223 BTSP already loaded into PMags, 200 rounds of .44mag (.240gr SWC), 300 rounds of .308 SP and 300 rounds of .45acp FMJ.
Typically all this is sitting in each of my vehicles locked away in a tote and out of sight. When I go to the range I bring a lot more ammo but some times that runs out and we're not done at the range so there's at least a little more ammo in the car to use. When it gets used it gets replenished that day. Same goes for hunting trips and camping. I've had the unfortunate event of falling into a swamp in a particularly warm gun deer season here, gun and all, and my ammo that was on me got soaked. I was happy to be able to return to camp, change into dry clothing and realized that I had a supply of usable ammo in my truck.
I suppose though that in a SHTF or inclement weather/stranded situation any of that ammo could come in useful depending on the guns I had with me.
Really though I'm of the mindset that if I'm out and about and need to defend myself with my carry guns I'm going to give myself enough time and cover to retreat BEFORE I run out of what ammo I'm carrying on me. If I make it back to my vehicle I'm bugging out of there unless the vehicle itself is disabled too. In which case all bets are off. For me to return to engage hostile forces is both idiotic AND illegal.
This is who we are, what we do.