It is a real compromise. Try to burn in with what you like listening to most ...and that will be best.
If you burn in with Jazz music then rock will sound funky.
If you burn in with a computer signal then all music will sound digital.
If you burn in with classical music then rock guitars will lose their edge.
If you burn in with rock music then classical music will sound harsh.
Just remember that the cables can get musically stiff during burn in and sometimes left or right channel can get polarized. I suggest thoroughly flexing the cables once each hour (wrap cables around your ankles and see Jane Fonda's work out video for how best to do this) and, above all, don't forget to swap cables from left to right channel every 10 hours to avoid stereo image drift.
LOL ;-)