I limit/normalize at the final step to -.1db , and release a lot of music online via MP3 and the original WAVs. No fidelity issues. Are you sure your process is correct and that the final file is actually peaking where you tell it? For example, I open the final WAV in Soundforge and run a quick analyzer on it to verify the peak and RMS values to be sure they're close to what I'm seeing in the DAW's meters. They are not always an exact match, the fault of the DAW (Sonar). But knowing this and checking for it eliminates any baked mixes getting through.
Fwiw - while doing track level work and/or at the first couple mastering stages, I obviously leave more headroom than that.