In my experience, if a state charity regulator gets wind you're soliciting in that state without registration, they'll send you a letter requesting you to register. This may include for back years. But, in general, I don't think they want to bust you, they just want you to get compliant and get your fees.
If you're definitely actively soliciting in all the states requiring registration, use a vendor. But not all vendors are created equally and I've heard HC merged with another shop and clients have been departing. Granted, I heard this from two other vendors so draw your own conclusions.
If you're primarily in a handful of states, you may want to register in those jurisdictions but evaluate additional registrations on a case by case basis. After all, if you register everywhere, you're on the hook to pay in each of those states and if you become inactive, you need to go through steps to "unregister". Some vendors have attorneys on staff to make legal recommendations and others have folks that can't make legal recommendations but will offer their perspective based on years of experience.
Ultimately, as with most things FS, this is a question of risk tolerance.