I'm wondering if anyone can confirm this improves the success rate of the first layer by doing this before starting print. My case is using ABS with a Reprap. In may work on other 3D printers and materials.
Heat up the bed and extruder to the starting temperature long enough to stabilize, so that the printer sets a more accurate z-height at the start. While your printer may wait until temperatures are reached, it will usually start immediately without waiting.
After the extruder reaches starting temperature, wait until the filament material stops extruding from the extruder. Remove extruded material. This reduces failures due to material extruding before the print, but after the start, ending up in the wrong place.