Stock ROM before custom rom

[grashoper]

Just wondering. Alot of people (if not all) suggest that ya flash to a stock ROM (for your particular carrier I'm assuming??) and then flash to a custom ROM of choice, followed by one hard reset... all sound good so far?
After already running a Custom ROM like myself, i want to try out a new one, and if i don't like that one, back to Stock ROM and then new Custom ROM of choice... Still sound good??
So, my question is this. When flashing back to the Stock ROM, (after the flash has completed) and is booting up for the first time... how far into the setup procedure is necessary, before flashing another custom ROM? Do we have to let the carrier customizations run, calibrate the screen, etc. or can we just interrupt it with a reset, and force a boot-loader screen at some point and flash to custom ROM? Reason i'm asking is purely for time saving. It SUCKS sitting here waiting for all that to run thru before playing with a different ROM. Oh ya... I'm running the CDMA Diamond. On Bell
Thanks for your input.