With a 24 bit mix it makes no difference whether the mixes have a level of -0.1 dbFS or -10dbFS. I recommend a level of approx – 3dbFS . This, however, should only be taken as a guide. As long as the level remains below 0dBFS everything is ok. All master bus effects that are there to purely to increase loudness should be removed before the mix down. This applies to all types of limiters, enhancers, exciters, clippers, etc. In contrast, a moderate amount of compression on the master bus can be part of the mix and, indeed, should remain in the mix if it is being used to create a particular sound feel. Also, if, you have used a master bus compressor or a certain effects chain that has shaped the sound considerably from the very start of creating your track, it often makes no sense to completely disable them for mastering. This is because the track will probably sound quite different without them. With tracks like these the final limiter should not be used, so that the mix still has headroom. If in doubt, please give me two versions – one with master bus processing and one without. We can discuss and decide which the best version to use is after checking the material.