So this is just my opinion...
If it's me, I'd rather know so I can fix it! I go back through my old posts from time to time, and find all kinds of mistakes, so it's not like I'm all high and mighty about it. I would rather know. You can tell me.
But when it's other people... it's hard, when we're not standing in front of each other, (body language, tone of voice, and all), to convey the spirit in which correction is offered. So most of the time I don't say anything. Unless -
I feel like I know the person pretty well.
It's a glaring mistake that everyone else will notice.
They're writing for a large audience. Because they need to know better.
Though I do have my pet peeves.
Apostrophes. Learn the rules.
Try and/Try to. You try to do something. If you try and do something, you're doing two separate things; trying and whatever else.
Super. It's usually an adjective, rarely an adverb. In other words, "I had a super day," may sound all Leave It To Beaver but it is used properly, while, "Did you see that super cute shirt/guy/puppy/etc?" makes you sound like an adolescent girl whose carbonated hormones are affecting her mental capacity.
Loose/lose. To loose something is to set it free. To lose something is (most commonly) to be unable to find it.
I've got more, but that's enough for one day.