Agreed that heating up the bed before G29 is important due to thermal expansion. But the hot-end never touches the bed during leveling with the BLTouch, and is therefore not involved during the auto-bed-leveling process. So no need to heat up the hot-end in advance of bed-leveling.
Of course the thermal expansion of the hot-end might be a factor when determining the correct Z-probe offset for the BLTouch -- but that is pretty much a one-time measurement determined when installing the BLTouch.