Photomatix recommends processing RAW to TIFF before loading images (as a first step, before aligning, deghosting, etc.). That recommendation is based on the pre-processing to remove any/all artifacts from your individual images--because those artifacts will compound as you're pushing saturation, contrast, etc., in the HDR process.
So if you have lens distortion, chromatic aberration, fringing, flare, vignetting, etc., you want to address that before you move to HDR processing. Ditto with any white balance correction you need to do. As I primarily use HDR on outdoor shots, and often desaturate (even to full BW), I don't need to do a lot of white balance correction.
For me, when I need the intermediate step of RAW->TIFF prior to HDR is, as others have pointed out, when I'm stitching panoramas. You must-must-must stitch first--you can't just HDR your individual slices and then stitch the HDR-ed images together. And, if it isn't obvious, you need to crop your stitched panos before the HDR process, because if you try to process an image with the funky borders from the stitching process, it'll throw off all the contrast and DR adjustment.
I've been using Photomatix for almost 9 years now, with 'normal' images and IR, and I've tested individual projects letting Photomatix use the original RAW files and using TIFFs processed (by both ACR and DPP) from the RAW files with no adjustments. I can't tell any difference, so I believe Photomatix's RAW processing is just fine for my uses. I do tend to pre-process in DPP when I'm using IR images that I want to end up BW, because the little bit of color that the RAW files have can produce some odd/jarring results in Photomatix. On occasion I've tweaked the TIFFs in Photoshop (using Nik Silver Efex, applying red filter) before porting over to Photomatix, but that's a lot of work--the main benefit being smoother skies without weird artifacts.
I'd say the biggest thing, if you're doing HDR landscapes with a wide angle, is to address vignetting. If you're going for a very "HDR" look, any vignetting in your RAW images can become very pronounced.