If everything were tagged "#protip" then it wouldn't mean a damn thing.
This should be reserved for only the greatest, most practical tips. Tips that everyone should know. If they want to be a pro. Preferably, keep it concise - something that can greatly benefit someone without great effort or months of educating themselves on a subject.
(It wouldn't simply be a "tip" if it were a whole textbook of information. No doubt, people can greatly benefit from reading textbooks, but you wouldn't call it a tip.)