Unless someone else is really crowding you during your solo and playing close enough to be picked up by the mic, I don't think it makes a difference. However, after your solo, what are you doing with the mic for the rest of your show? Does it stay on? Slip into a pocket? Get turned off? If you have to move it when you're done, make sure it's somewhere you can easily get it when you're done, sometimes the bell on a marching baritone/euphonium can be tricky to maneuver around. Out of curiosity, what are you performing the solo on? (marching bari, marching euph, concert euph, etc.)
1905 Boosey Class A Euphonium-Wick SM4M
Yamaha 301M Marching Baritone-Schilke 52
1960 Conn 11J-Conn Helleberg
1961 Conn 14J-Vincent DFL
2015 King 2341-Bach Corp. 24AW
Olds O95 Sousaphone-King 26