If you have full edit privileges, anything that's a clear improvement is fine, even if it's small. Please feel free to fix small errors, especially in newer posts or prominent old ones.
But one thing to be cautious about: make sure you try to fix everything there is to be fixed. It's unfortunately common to see edits that fix a glaring mistake in the title ...
Edit the existing one to be right, or as right as you can make it.
This expectation is built right into the system: editing an answer (a) bumps the question to the front page, and (b) allows previously-locked-in votes to be undone. So if anybody is unhappy with the revised answer, they can flip their previous votes.
Of course, if it's totally unsalvageable ...
If it conflicts with the original authors intent, such an edit should not be accepted.
That being said, I wrote the edit thinking it accurately reflected what the author had intended but had difficultly communicating technically.
The word hydroscopically was used when the word he probably meant was hydrophilic. These words are very similar. He ...
I'm under the impression that any and all edits for spelling, format, and style are accepted and encouraged (with the exception of swapping between acceptable UK-US variants). You're likely seeing some combination of people hunting for their editing badges or just being a little particular.