...before you show it to agents or publishers.
I heartily recommend this method for catching typos, awkward wording, poor grammar, word repetition, voice adjustment (that is, the "voice" of your manuscript), finding inconsistencies, and much much more.
When I do it sometimes I get all dramatic and pretend I'm auditioning to read my own audiobook. But that takes longer than reading in a monotoned rush, so I usually fall back into that. Either way, it works wonders.