I not familiar with the FHA "90%" rule you mention, but I think USPAP will override, in that even if it is 95% or more complete, you must have enough data, be it plans, specifications, contract, selection sheets, builders verbal but documented representation, or an identical model, etc., to complete the appraisal.
Even VA is flexible, and allows the completion of the appraisal w/o plans and specifications once the property reaches the "buyer preference stage". VA does not place a "% complete" threshold on what this stage of development is, but DOES require that you utilize a credible source for the information on how the house will be completed, and document that source.
Unless you are certain about the FHA 90% rule you mention, I see no reason to require plans and specifications, as long as you have a good source for what the finishes in that last 10% will be.
Keep in mind, however, that FHA DOES require plans and specifications, even for a 100% complete but never occupied home, depending on whether the builder provides certain warranties and is an FHA approved/certified builder. It may not be your job to obtain them, and you may not actually need to have or use them to complete the appraisal, but ultimately they will be needed for many FHA loans packages on new construction.