Is it bad etiquette to come to office hours that aren't for your particular class?
I don't think the professor minds too much, but you should of course be prepared for him to decline if he chooses to. Though I myself could hardly care less, I have colleagues who would definitely think unsuitable (because it's not how it's planned).

However, I think the other students might mind, as you noted, if they began to see it as you “stealing” time from their limited allocation. As always, sharing is no problem unless the resource is scarce :)

So, in short, I don't think it's a big problem, given that you are mindful of others.