I don't use the tremolo much, but I do believe it adds an airy element to the tone, so I tend to fix it down it in Eric Clapton style.
I have tried steel and brass blocks of various sizes but I haven't found it add or reduce tone in any big or meaningful way. The only difference I found was when swapping a really cheap cast trem from an import guitar for a 'proper' one. It added a little (and only a little) of body to the tone, but that could have been a combination of fit, quality, saddles, the trem itself as well as the trem block.