-6dBFs, that should be my 'mental' limit when mixing?

If you send the mastering engineer a 32 bit float wav file, then the peak level will not matter. this is because floating point wav does not truncate bits the same way int wavs do. Also, the less EQ on bass, the better, because these frequencies are delayed more when eqing.

That's total bollox. You can alter the total volume numerous times without affecting the quality or introducing artefacts. That is, if you use an audio editor that uses a format with more than 16 bits. There will be differences, but these are at around -140dB. The typical noise floor of a "mastering grade" DAC is at -120dB. Any mastering engineer worth it's salt knows that.

I hear all the time that mastering engineers demand all peaks to be below -6dBfs and reject tracks e.g. peak-normalised to 0dB. In practice this should be no problem at all for the above mentionned reasons. They just want to make sure you haven't clipped it.

Exactly, you can do a simple test of inserting a few gain plugins, one set to -100db, one set to +100db, do that a few times and listen, you won't hear anything. Even if you do an analysis the distortion will still be well below what would make an audible difference in most mixes.

I've never demanded that as it makes no difference, it's a waste to normalize but still won't usually have any big consequence. But yes clipping is the only thing that's going to be an issue in most cases.

This is normally how I recommend people approach mixing to make mastering easier later on:

http://tarekith.com/assets/pdfs/Mixdowns.pdf

anyways, thanks guys I think I got a good idea of what I should do.

That's a good document. A lot of it is common sense, but there is a lot of useful info in here.

basically you try to just mix your music at a good level to make things easy I try to go for about -6 db it does not matter if you have to turn down the volume jsut leave the mastering to the mastering engeneer