The general concensus is that you can so long as the end result is clear and legible. Some people use little bits of white sticky paper. I use a tippex mouse. But the best solution is to do it in pencil and either submit it in pencil or ink over your final answers.
Well, yes, it is a chore but it does give you chance to check your answers before you send it off. I have found a small word I have missed on more than one occasions by doing this.