Engine offset is important to help maintain over all stability. The offset I'm talking about is pitch (up and down) NOT yaw (left and right) Most planes require very little offset and you should refer to the building instructions. However if they offer no suggestions mount your engine with a degree or two of negative (engine pointing down) offset. You can always adjust it by adding washers to the back of the engine (one on each side to start with) to give more negative offset. For more positive just remove them from the back (or add to front)
How to tell which direction to offset. Start with the plane flying and trimmed out at about 1/3 power or enough to allow it to fly with ease. Level the wings and nose so its flying straight and level. Quickly throttle up the engine and watch what the plane does. If it climbs then chances are you need a little more negative offset. If it dives then maybe a little more positive. The closer to zero degrees of offset you go chances are the plane will fly faster but may lose stability in turns or climbs if offset is too positive. It may feel like the plane wants to balloon or fall over on its back.