At the risk of sounding like a crusty old bugger (which I am!) I would like to emphasise that the correct use of English (presumably your native language) is important. Believe the "leave it to later it's not important" merchants at your peril.
While you might get through OASC with flying colours, when you get to IOT you might just get a crotchety flight commander such as me who sets great store on using one's own language correctly.
If the take for aircrew is as tough as it appears to be nowadays then the person who can write English properly is going to cream it over the buffoon who can't.
When I was at IOT (Main and Recourse Squadrons) there was more than one cadet that I gave a hard time to because of their crap writing abilities - and there were 2 who admittedly were having a hard/borderline time with the leadership exercises but their written communications was the deciding factor in me chopping them. Presumably they are now stacking shelves somewhere instead of playing with one of Betty's wonder machines.