I'm not sure what you mean by not straight. Do you mean not straight when you are tilting the blade 45deg, or not straight making a 45 deg miter?
My assumption is that your not getting a good 45 degree miter? This will happen to me on my Craftsmen table saw if I am using only a miter gauge set to 45 degrees. This happens because the wood is being dragged on the table. Because of the drag, the wood is slightly moving from its original location. This results in cut that is a not straight.
A 45 degree table saw sled (jig), or a clamping miter gauge, will solve the problem.
On a miter saw you can get a bad 45 degree miter because the wood will move slightly when making the cut. Hold the wood tightly or clamp if necessary.
If you're cutting white baseboard trim, caulk will fix the bad cut:gar-La;
Dan