I am no expert, although am type rated on the
737 - 300/400: Our teaching was that the Pitot heaters are selected to on as part of the after engine-start flow. No reference to outside conditions:
~Set Flaps.
~Genny 2 then 1 on line.
~PITOT heaters on.
~Eng Ant-ice on if reqd.
~Packs On.
~APU bleed off.
~Pressurisation to FLT.
~Engs to CONT.
~RECALL checked.
Read checklist, call for taxi.