I've spent the last couple of days putting together submissions for various poetry competitions - re-reading older poems and re-writing more recent pieces. I can always find something to correct or improve and, I guess, in this sense poems are like children - we're never really done with them. It's worth taking the time to read work over, even poems that you know well, have sent out before and whose spelling, grammar and syntax you would happily vouch for in a court of law. There is nothing more galling than sending out a manuscript and only spotting the errors after a trip to the post office when it's winging its merry way.
A mistake won't necessarily ruin a good poem but it may suggest to a judge, editor or publisher that the writer is sloppy, doesn't pay attention to detail or is not serious about the work. If you don't care enough about the poem to present it correctly, why should they?