I have had this discussion with Dan Morgan (a very good player who talks a lot of sense) on these forums in the past, during which he argued that assuming opponents are competent until proven otherwise is a huge leak. I think he is completely wrong in both a philosophical and a practical sense. But it's up to you to decide where you stand on this issue.
fwiw I think 'huge leak' was a bit strong. Definitely wouldn't agree with myself now

Another thing that I didn't properly account for at the time of writing that was that the majority of my poker experience comes from HUSNGs which I'd say is a format that lends itself a little more to people doing really spazzy stuff than say 6max cash. I guess that is very hard to quantify though since you don't play many HUSNGs and I don't play much 6max cash.
I'd probably alter the sentiment now to saying that it is better to assume that everyone plays a static strategy rather than everyone is terrible which I think is closer to the true population tendency.