I'll talk about what I try to do, and let others voice their opinion on its suitability or unsuitability:
1) I always try to fix spelling errors in titles, even if its the only thing wrong. I think clean, clear titles matter.
2) Since this is a permanent repository, I'd prefer to have good spelling and formatting in posts, but I think it's pointless to fix one spelling error, unless it's a key word someone might search for and it's only used that once. If I do edit, I try to fix absolutely everything I can at once - every bit of spelling, grammar, formatting, unclear expression, ambiguous abbreviations - the lot. (I don't always succeed - e.g. I sometimes spot something important immediately after I finish editing, to my annoyance; in that case, I figure if I re-edit immediately, at least it only bumps it while it's still at the top of the active queue. If there's only a trivial problem remaining, I do try to leave it alone.)
3) If I am prompting the original poster to add something, I will sometimes mention other things they should fix in their own edit.
Here's the principles that I think are important:
clarity/comprehensibility and being able to find posts matters
one or two minor spelling or grammar errors alone that don't impact the first thing aren't a big deal
if you're editing anyway, get everything you can, including stuff that doesn't matter (not least, because then nobody else will be tempted to edit it three hours later)
try not to keep bumping old posts during busier times. If you're editing three six month old posts each four times in four hours, you should maybe slow down and give the new posts a chance. (Occasionally, I have caught myself re-editing an answer or question on an old post more than once in a short period ... and then I hear the @gung-angel on my right shoulder whispering to me to avoid temptation and spare the front page)
To respond to your question more directly, I think you should fix titles, and you sound like you're doing just fine.