Hi ADKstorm,
Which joint would you be referring to? Actually, the "default" root opening would be per the qualified WPS or the prequalified joint designation(they can be different). For instance, a prequalified TC-U4a would have a 1/4" root opening with a fit-up tolerance of +1/4,-1/16(for the 45 degree bevel using SMAW), and a prequalified TC-U4b would have the assumed 0" to an 1/8" root opening like you are referring to with a fit-up tolerance of +1/16,-1/8.
I wouldn't just assume the root to be zero unless the joint designation is as such.
(I was assuming you are working to D1.1-Structural Code)
Hope this helps,
John Wright