An acceptable amount for pattern matching to be "off" after a trip to the tailor?

None.

I have probably obtained a dozen alterations that affected the previously-perfect alignment of fabric patterns, and in nearly every case the pattern (simple stripes usually, but the occasional plaid) was aligned when I received the garment. It's just my two cents, but in the vast majority of circumstances, there is no legitimate reason that a proper alteration requires a pattern disruption.