The use of the mixtures in general has to be always with consideration. Of course it belongs to Organ Pleno registration and could be used in playing free works from the Baroque period, such as Preludes, Fugues, Fantasias, Toccatas, Passacaglias and Chaconnes etc. Also mixtures can be like a crowning jewel of the organ sound in other periods as well (provided it is well voiced and tuned).
Always check if the mixture is a low one in which case you would need 16' in the manual as well. Don't forget that Organo Pleno registration simply implies principal chorus. This means you can build the Pleno sound without the mixtures as well by using the pyramid of principals and mutations 16', 8', 4', 2 2/3', 2', 1 1/3' and 1'.
This might be useful if your mixtures are too harsh or the piece is of a gentle character or key (perhaps some preludes in F major etc.) or if you need variety in your recital and you are playing several pieces which would require principal chorus registration.
In German Romantic tradition, when you build crescendo, the mixtures come after softer reeds of the Swell division but before the stronger and more powerful reeds of the Great. In French Romantic tradition, the mixtures come in after all the reeds.
A final thought: don't make a mistake I once did with the mixture of the Great on my organ at Vilnius University St. Johns church. This time I was playing a prelude in C major, BWV 547 by Bach and I thought I needed a mixture with the third sound (which would work really well for middle German Baroque). But this mixture consists entirely of repeating octaves and fifths. So I added a Tertia stop at 1 3/5'. Since this third is non repeating, the result was rather too harsh.