You definitely need to master them if you want to make more than one tracks.
Mastering has several levels:
- The basic level should be making the tracks similar. At least, you need to make the music the same loudness.
- Next level, it's time to fix the issues you missed during mixing. Say, LR balance, dynamic balance, etc.
- Even further, you can adjust the tonal balance and others, just to make it a bit better.
Relax, it's just a jam, a game, a chance to have fun. Don't be too stressed. Do whatever you could.