I think it's ok to fix typos, but not spelling mistakes.

That is, if a poster intended to type "weird" but mis-typed it as "wierd", it's ok to fix it - and the poster ought not be offended. But if the poster intentionally typed "wierd" because (s)he thought that was the correct way to spell that word, then you may well cause offence by correcting it.

I often like to quote some text from the post I'm replying to - for context, mainly. On occasions, the text that I have wanted to quote has contained a typo (or spelling mistake) and I've often wondered how the op feels about seeing that error italicised at the begiining of my post. Does (s)he think I'm trying to belittle her/him by highlighting the error ?

I tried to find a typo in your post that I could quote back ... but couldn't spot one, dammit :-)

Good luck !!

Cheers,
Rob