It's a good idea to remember that even if you've been stung before with no allergic reaction, that doesn't mean you aren't allergic.
Often, people will only show symptoms on the second sting, because the first one is the one that primes the body to overreact.
So people don't realize the danger that beestings or yellow jacket stings can pose until they have a second one and suddenly get really sick.
So, I would treat every sting as serious unless you know from long experience that they are not. And remember, even if you've been stung multiple times, there is still a slight chance that you could have an adverse reaction.
It's not something you should take for granted.