So I just can't seem to finish my first plane......

I mount my engine and it seems like there's a bit of left thrust, ie it's not exactly straight rather it seems like its pointing a bit left. I read that a bit of sidethrust isn't a problem, and that it takes several degrees of thrust to cause a problem in flight. First of all, is this correct? Second, how must thrust would cause a problem? And third, How can I fix it if that's neccessary? I was thinking of putting some washers on the left side mounting screws. Of course I'll have my plane checked out before attempting to fly it [with the help of an instructor], but I want to know what I might have to do.] Thanks!