WC,
It's a "trick" to keep the cut surface flat and to get two cuts to line up when your bar is not long enough to make a single-set cut. Two things that will inprove your chances are a properly dressed and squared bar and a carefully sharpened chain. If the rails of your bar have worn more on one side than the other, the chain will run slanted to the "low" side and make it nearly impossible to get a straight and flat cut. So too, if your chain's cutters are not balanced for sharpness and depth set, they will tend to pull the cut off-line. If these are your problems, it's time for a bit of maintenence on your bar and chain. Check your bar with a good square to see if one of the rails has worn a bit more than the other. It they're not deadnuts square to the side of the bar, the bar can be lightly dressed on your bench grinder with a flat tool rest followed by a fine file to remove any formed burr, but be careful not to grind the points on your nose sprocket.
If you file sharpen, it's very hard to get all cutters precisely even in pitch and angle and length. This is fine for cross cutting. However variations between the two sides of the chain will cause it to pull to one side or the other and make things difficult in a rip-cut. A careful regrinding on a good sharpening rig will balance out the cutters so they'll run straight and true. I put a bit of care into my chains and actually mic the cutters when switching between sides to make sure they come out precisely even. Makes a big difference in the tool.