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There's a lot to say about mastering, so here I'll stick to just one point. And that is if you plan on taking (or sending) your tracks to a mastering studio, then you shouldn't have mastered them yourself already.
There is a subtle dividing line between mixing and mastering. You might say that mixing should all be done in the individual audio and auxiliary tracks and that nothing further should be done in the master track (or master channel if you prefer to call it that). This is 'pure' mixing with not a trace of mastering involved.
It is however common that such a mix will not have quite the overall balance of frequencies that you would like. You could tweak all of the individual channels, but that would take time. Or you could put an equalizer into the master track. If you do the latter then you have now dipped your first toe into the mastering process.
You might further consider that the dynamic range of your mix is too great and insert a compressor into the master track. You might think your mix isn't hot enough so you put a mastering limiter into master track, or perhaps a 'vintage' compression plug-in. You are now definitely mastering your track.
The problem is that any mastering that you do yourself will put a limit on what can be achieved in the mastering studio. You will be asking the mastering engineer, who probably has many years of specialist mastering experience, to work with one hand tied behind his back. If you applied a little EQ to the master track, that shouldn't be a problem, as long as it was a straightforward digital EQ with no warmth or other artefacts. But if you did any dynamics processing or harmonic generation in the master track, then that can't be undone in mastering.
The whole point of having your work mastered is to get an expert to work on your music. If you place limitations on what they can do, then you won't get the best value from your session.
In summary, a little clean EQ in the master track won't affect your mastering session to any noticeable extent. But stay clear of compression, limiting and warmth in the master track. Your mastering engineer can do all of those to a much higher standard.