I agree with the hierarchical errands context notion, and use it all the time. For example, some grocery items I'm willing to buy at any of the stores I patronize that sell groceries, so they get dropped in the Errands : Grocery Store context. Other items I want to buy at a specific store might go in the Errands : Target or Errands : Costco or Errands : Trader Joe's contexts. And finally, there are even less-specific purchases that would get tossed in the top-level Errands. I don't choose to make Target, Costco, TJ's as subcontexts of Errands : Grocery Store because I buy non-grocery items at them as well and don't want to either duplicate them (which would complicate OF's quick recognition on entry of a few characters) or have them in the "wrong" place. Your mileage may vary here; I'm using a laptop so I can have a perspective for pulling the separate contexts together, and if I was using an iPhone (thus no perspectives) I would possibly prefer to have them nested in there as Errands : Grocery Store: Target, etc. and rely on the location awareness support to remind me that even though I went to Costco to buy bananas, I could also pick up that new flat-panel TV as long as I'm in the neighborhood.
In my opinion, if having a "temporary" context to remind you when you're near some location that you could take care of something will help you get it done, you should do it. You can always delete the context after you've marked the action complete if think you won't need it again. If you think you might need it again, but not soon, you could mark the context as dropped and you won't see it in the usual list of remaining contexts. To mark it as dropped, select the context in the sidebar, then ctrl-click or right-click to bring up the contextual menu and select Dropped. There's no reason the list of contexts has to be carved in stone when you start using the program, never to change...
In my opinion, if having a "temporary" context to remind you when you're near some location that you could take care of something will help you get it done, you should do it. You can always delete the context after you've marked the action complete if think you won't need it again. If you think you might need it again, but not soon, you could mark the context as dropped and you won't see it in the usual list of remaining contexts. To mark it as dropped, select the context in the sidebar, then ctrl-click or right-click to bring up the contextual menu and select Dropped. There's no reason the list of contexts has to be carved in stone when you start using the program, never to change...