thicknesses are usually pretty consistent
However, if you are gluing boards edge to edge to make a wider panel, they may have some curves. You will have to joint the edges or straight line rip them for the best results. Always pick through the boards to find the straightest by sighting down the length first. Some boards my be slightly cupped or twisted even though their thickness is uniform.
If you are joining boards face to face as in a workbench top, then you have probably ripped them to the width you'll need for thickness of the top. The ripped boards will be the same dimension, but unless they are straight you will have slight offsets when gluing up. Jointing them straight will be best here also.
A curved board ripped on the tablesaw will have a slight curve as did the original. A straight line ripping jig is the only way to insure a straight edge.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
Last edited by woodnthings; 09-04-2013 at 04:06 PM.