None that I aware of if you are putting the entire vessel in an oven. On the other hand, if you are doing a local PWHT, there is specific guidance provided in WRC 452 that I would consider "good practice".
Read UW-40 of Section VIII, Div 1. If you intend to perform a "batch" post weld heat treatment, thermocouples must be placed at specific locations to assure adequate temperature. The reference to this paragraph is UW-40 8 (c).