Generally it is not advised to use preview files for final export.
thank you! for some reason i thought it just saves another renderring.
Steve Hoeg can correct me if I'm wrong but consider a situation where you have a 29.97i sequence and some stills, which are progressive by nature plus some 24p material, with a 1/2 quality preview and you want to export to a different format, be that resolution-wise, frame-rate, or whatever, then the question arises should you use the already rendered previews with the inherent quality loss, or use the original material? Since you are going to lose quality anyway, my feeling is to keep the loss to the minimum possible by using the orginal material, not the preview quality.
Do you know what quality the preview files have? Were they rendered to 29.97i, at what resolution, how were progressive stills handled, what happened to the 24p material, how was it interpreted, etc.
My suggestion is to not use previews but use the original footage for your final product. The rendering time may be longer, but isn't it quality that counts?