It depends on what you mean by "reasons not to declare". There may very well be repercussions and at least a preliminary investigation even for something as innocuous as telling ATC you have a fuel gauge malfunction and would like to land to check it out. You do not even have to declare an emergency to trigger the "repercussions". If you are in the system and divert for reasons that might be airworthiness or medically related, ATC is supposed to file paperwork and an FAA inspector will almost certainly come a-calling. If you declare an emergency for something airworthiness related or medical, even if you land at your filed destination, I would expect the feds to ask some questions. If it's possibly the result of poor ADM or airmanship on your part, ditto, though I don't have any personal experience so far with that kind of scenario (knocking on wood).
But the question is, if you or a passenger might need the fire trucks or medical assistance when you land, which is more important, ensuring the safety of everyone involved, or worrying about the suits and the paperwork? Skin, tin, ticket is the priority list I was taught. That said, if there is NO upside to informing ATC of what is going on, and no way they could help you either on the ground or before, I would not give them more information than they need to know. By definition, of course, if you have a true emergency, then you are likely to need help from someone as long as there is a chance that someone on board will survive.