I must point out that, like any other mantra, JFDI must be taken with a grain of salt. Think about the following scenarios. You meet a guru who's willing to advise your startup for a mere 5% of the company. You find an investor who's willing to put in big money, given you tear up your term sheet and use his unconventional terms. You think up a feature that, while consuming vast dev resources, could result in total market domination.

It'd be absurd to JFDI on any of those, not without doing the proper due diligence. For day to day decisions, JFDI is absolutely the way to go. For decisions that could kill your startup, BJAPC (Be Judicious And Proceed Cautiously). Admittedly, that's not as catchy.

edit: formatting.