Most all ARFs these days already have built in thrust angles based on the recommended engine sizes and other factors. Deviations from the planed engines, will call for slight changes in thrust among other things, but if you stay within their recommendations, you should be fine. Even if it is a little off, it won't be unflyable, it will just require more trim and quicker responses on fast power changes.

If you can't tell by looking, after you have your engine mounted and the prop installed, measure from each prop tip back to the back of the vertical stab (not the rudder). The measurement on the right side should be slightly less than what you get on the left, if the factory set it up for you. If they are the same, just add washers under the left half (about 2-3 deg is good usually). I don't use down thrust on a plane with symmetrical wings