Yes, but not for Bluray. There is not much processing done by PC for bluray source. Its 1080P, meaning, you are not scaling the resolution at all. Its a progressive source, so no de-interlacing is needed too. That's the reason all bluray players picture quality is almost same, if they output 1080p24.
The HTPC image processing comes into picture whenever you are scaling/de-interlacing the picture. Meaning, 720p, 480p and other sources than 1080p. You can also use ffdshow which has many many filters, each for de-interlacing, scaling, sharpness, deblock, noise reduction, colorspace correction etc. Gives lot of control and options. The bad thing is - with all these options, we are never done tweaking. Not to mention the updates, hardware driver updates and changes. ATI and Nvidia drivers are notorious to have one bug or the other. Read on forums about those before diving in. I have built the HTPC twice, used it for some time and gave up on it. I still have one my rack, rarely used now-a-days.