Is it OK to calibrate a camera using effects and filters such as increasing contrast and brightness etc. to improve the detection of corners of a chessboard? Even if the used mode for detecting the chessboard is just plain gray scale?
Even though adjusting contrast or brightness is probably ok, you should be weary of tampering with your calibration images. Turning on more lights would be a better way to go. On the other hand, definitely do not use any kind of filtering or sharpening, because that will change the position of the corners.
You should not experience any major drawbacks. Generally such techniques utilize adaptive thresholds and rely on gradient orientations (especially normalized ones), which are intrinsically invariant to uniform illumination and contrast changes. In other words, brightness changes will not effect your subpixel positions, if you are using an appropriate corner detection algorithm.