First and for-most, do what ever the directions tell you to do! In most cases if the directions do not say anything about the thrust angle it should be set to 0-0, or 0 degrees down and 0 degrees left or right. In some cases the kit will need some down and or right thrust to the engine, 1, 2 or 3 degrees is common. This helps offset the left turn tendency of the aircraft.

In short the answer is... if the directions don't tell you do do it, don't. mount that sucker straight on! After you fly it for the first time, if you want to play with it go for it, but first try it the way the manufacturer recommends!