I have - so far - a mixture of Liesl & Co., StyleArc and Cashmerette patterns.
A pattern is a pattern and it doesn't really matter whether the designer used metric or imperial. You use/start with whatever length the pattern happens to be. Seam allowances are typically 5/8" or 16mm. Actually 5/8" = 0.625" = 15.875mm. The difference of 0.125mm is barely the thickness of a thread and would be swallowed up by the thickness of the cutting line - not to mention your ability to cut fabric that accurately.
If you are used to using imperial measurements go for one marked in inches, eigths of an inch etc. If you are used to working with metric measurements go for a metric one with cm and mm. If you are bi-lingual with measuring (bi-measural?
) then it really doesn't matter. Just be sure to use the same system if you are trying to match two different curves for length.