I meant "should of", especially in writing. I am not sure if "should've" is supposed to sound the same as "should of", but seeing the latter in writing annoys me a lot for some reason.

They sound the same in most American English accents, at least.

(This response contains another funny modern colloquialism: 'a lot' gets used by both native and non-native English speakers where 'many' or 'greatly' is prescribed. Not that we need to be prescriptivists, of course.)