Generally speaking, you don't really *need* to.
If you have a private tutor for english already, I would suggest asking him/her to read over your work once every 2 weeks or something to give you feedback.
Like a few people have already mentioned, you should get as many people to read your work as possible. Friends, teachers, relatives.
Personally for me, before handing over my final work, I paid a qualified teacher/4U marker from a different school to get some feedback (he basically took my work for the weekend and checked for *minor* spelling/grammar mistakes, suggested I cut down content that I felt I should've kept). Looking back, I wish I hadn't asked him for help, it was such a waste of $350... especially because my 4U teacher at school was the head teacher of the english department, and she was absolutely fantastic.
In the end, I didn't actually change my work according to his advice (as in cut down my work, which was short enough as it was).