Apologies for the slow response. Thanks all for taking the time to give your views. Thanks also to Bart for the link; although this might not be what I am looking for regarding ACR, it will definitely come in handy for Photoshop work.
Even though some interesting views have been given in this discussion, I still feel that the ability to change the clipping threshold levels is a strange omission by Adobe. On reading the responses, I believe that by stating a number (250) I inadvertently managed to direct the discussion away from the core issue I was raising. I am certainly not advocating that it is necessary for all pixels to be below 250; as others have said, if it looks good to clip pixels to 255 or 0, then do it. However, I still believe that it is beneficial to ensure that areas where detail is required in the highlights (e.g. white woven fabric) do not get near 255. I'm not going to give any numbers as they are obviously not rigid and what appears white in an image is affected by the tonalities and contrast in the image as a whole. All I want is a quick way to verify that areas of an image do not exceed what ever level I decide on for white (or not white, as the case may be); surely others find it useful at times to be able to check that areas of an image fall above or below a certain value, not matter what that value.
I appreciate what Henrik and DigitalDog have to say but, in my workflow, I am hardly ever involved in the process of taking an image to print beyond delivering the files to a client. From experience, I know that the CMYK conversions are often not done well and, although I guess you could say that this is not down to me, I want my images to look good when they are seen by others, who will not know or care that my images were delivered looking good only for them to look bad on paper through poor conversion. Given this, I do like to leave a little headroom in areas where I want detail.
Finally, regardless of my methods, in my opinion, good software design should be about enabling the end user. Adobe's decision to omit the ability to specify clipping levels does the opposite of this: as stated in my original post, Capture One Pro, Aperture, Phocus, Canon DPP (and many others) all have this facility and I know many other photographers and retouchers who make regular use of it and have incorporated it in their workflow, so I just wonder why Adobe does not provide it in ACR.