Up or down thrust is a different matter.
The incidence of the wing, relative to the tail, comes into play here. A flat bottom airfoil generally requires some positive incidence, while a symmetrical airfoil MAY be set at zero. Hopefully, the designer/manufacturer of the plane will have given the starting incidence settings. The decalage (incidence of tail relative to the wing) can be checked by putting the plane into a power off dive and noting which way the nose pulls. Adjustments to the decalage must be made before thrust is adjusted.
The accepted method for setting up and down thrust is to fly parallel to the runway and pull vertical and adjust depending on if it pulls to the canopy or to the wheels.
Ultimately, as Rodney pointed out, the plane should stay level when the throttle is advanced.
For a more complete series of changes to make in trimming a model, there is a pattern trim chart that works well at
http://www.wtp.net/DBEST/trimchrt.html
There is a heck of a lot more to trimming a plane than adjusting the Tx trims.