I usually promote at whatever the level my highest guy is at during the Balbazak battle (13 or 14), because I like my level symmetry, and that's a really easy grinding spot.
General advice, I'd say promote whenever there's a weapon you can't use unpromoted. Promoting immediately will gimp you in the short term with no real long term benefit, and grinding up to 20 kinda kills the point of a challenge run.
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SuperNiceDog won the Guru contest. Good job.