I recently have had a number of questions on and/or about this subject and i thought it might be nice to clarify a few things. Checking a knife for straightness can be very useful and is a very important part of knife sharpening. Here are a few notes on the subject-
-There are two knives of straightness one needs to be concerned with... warping from side to side and torsional twisting. Torsional twisting can sometimes be a problem and sometimes not (depends on the type of knife, grind type, and severity). Side to side twisting is almost always a problem (i say almost always because there may be a time when it is not... i just cant think of one off the top of my head).
-Checking for straightness is best done using your eyes, sighting down the blade in a variety of ways and from a variety of angles. However, it does take time to train your eyes to be able to see this well. Severe problems will be easy to see, but less severe problems can sometimes be very difficult to see. You will often sight down the blade spine from handle to tip and tip to handle, as well as the same with the edge side. It can also be a good idea to take a more straight on look down at the spine and edge. It can also help to look at the knife from the spine and edge, while holding it parallel to the ground with the tip facing to the right or left and the handle the opposite.
-It is not usually a good idea to test by laying the blade on something flat. This is for a few reasons. Most flat looking things are not flat. However, moreover, the grind on the sides of the knives will effect how this blade appears. For example, asymmetric knives may appear to be flat on one side while on the other the tip may appear to be bent upwards. This is an optical illusion and is a function of the grind and they way it lays on the flat surface.
-Slight warping on single bevel knives is sometimes seen. I had many discussions about this with very well respected professional chefs as well as knifemakers (sharpeners and blacksmiths). Server warping can be a problem in use and sharpening. Slight warping can be corrected through sharpening and does not need to be twisted back by hand (which i highly recommend not trying yourself... it is very difficult to do well and can potentially cause significant damage to the knife, not to mention you). Chefs and craftsmen agree that slight twisting does not effect performance in cutting and is best fixed by sharpening. More severe warping can be fixed by one of two ways... one will require refinishing the knife, but is the better solution. The other is more difficult and dangerous, doesnt provide as good of results, but can be done without refinishing the blade. I do not plan to share these on here, as when things like this have been shared in the past, many people attempt to do it and i end up seeing a lot of major problems that i need to fix. Sorry guys.
-Bending from left to right is often fixed with a tool used to bend the knife back. However, its not quite as simple as just picking the bent spot and bending it back. Like i mentioned above, when things like this have been shared in the past, many people attempt to do it and i end up seeing a lot of major problems that i need to fix. So i dont plan on explaining this in more detail to try to minimize this kind of problem. However, for those that have been to my shop, you can see me do this on a somewhat regular basis. This also requires great skill and experience to see well, and it has been my experience that fixing this kind of warping well also requires training and practice. I cant tell you how many knives i see for repair that people have tried to fix at home. Even when things seem to be fixed, there are often problems that the owner cant see. Anyways, this kind of repair is necessary to do before attempting sharpening. Sharpening a bent knife will cause issues with the bevel, profile, and geometry of the knife.
Again, i highly recommend not attempting this at home. Not only are the fixing processes potentially dangerous, but the chances of causing more damage than good are very high. I have spent a long time training in Japan to be able to do this well... not only in the actual repair process, but also in assessing the problems through visual inspection.
Hope this helps.