Both would be best. It all needs to be pretty straight, but 1/8 or so is not going to be the end of the world.

Im thinking on the fly here but if your fuselage is bowed probably the best thing you can do is draw a straight reference line on a flat surface to represent the centeline of the fuselage. Then center up the fuselage at the front and rear on the line. If its crooked it will be somewhat shaped like a banana compared to that reference line. The you just need to rig up a jig to force the fuselage in line with your refernce line and restrain it there. You might be able to just heat up the covering and straighten it if its just slightly bowed. If its worse you might have to remove the covering, dampen the wood and then holding it in a straightening jig let the wood dry out.

If its just a slight bend it probably wont really effect a little plane like a twist enough to make a whole lot of difference. If its 1/4" off then you might have some issues. I bet that you can work most of it out by bending the fuselage a little at a time and then while its bent, shrink the covering tight wherever it loosens up. Repeat that process until the covering wil not shrink anymore of the fuselage straightens out. That should cure it unless its severe. If its bowed THAT bad you can probably get some help from whoever you bought it from if its new.