One of the most celebrated violin teachers, Ivan Galamian, used to advise dividing your practice time into three categories: Building, Interpreting & Performing time.

Building time should be "devoted to overcoming technical problems and advancing one's equipment in general". This is time to find technical errors, come up with solutions, experiment with those solutions to find the best one, and then to use slow practice and repetition to entrain proper muscle memory.

Interpreting time should be "devoted to making the playing of the musical work conform to one's interpretive ideas".

Much of this work can be done away from the instrument so as not to let technical considerations influence interpretational choices. Phrase markings, dynamics, characters & timbres are all decided during this time.

Performing time should be "added whenever a piece is being readied for actual performance". You should always choose a piece that has been readied both technically and musically, before you subject it to performance run-throughs. Otherwise, you run the risk of solidifying poor habits. You can usually use a piece that is ready for mastery.