I would not perform any PWHT of 6061-T6. The reason is that any PWHT will result in over-aged material and a loss in strength. This aluminum alloy is artificially aged and any additional heat will result in coarsening of beneficial precipitates from PWHT.
Keep the heat local and watch your weld interpass temperature. If you need strength after welding, it would be best to re-heat treat the component - solution treat, quench and artificially age.