Political:
I have mapped for the Goldsrc/Source engines for
more than three years. For much of that time, I used to think it was fine to just "wing it" - never work in phases or plan in advance. That couldn't be farther from the truth, and I regret the years I've wasted on projects which inevitably
failed as a result. We are lucky in that "detailing" for the Source engine mostly just requires piling props and decals on top of things, as opposed to DOOM, Quake, or Half-Life 1 where everything is effectively part of the level geometry and can be easily designed along with the general layout.
Do not squander the fact Source map development is very modular. Do not waste time detailing maps as you go along, because it'll be a major hassle to undo those changes when you need to redesign areas. Your art style will come out
inconsistent and sloppy.
That's not to say A1 maps have to be hideous, though. Even when dev-blocking out a map I encourage everyone to alternate between teamcolors and grey for highlights - catching players' attention, et cetera. Even placeholder brush work can be designed to be fairly appealing. Early lighting, skybox ambience, and iconic props are fine too. Some things such as the ground can be textured and even displaced immediately.
But please oh please, don't completely rule out the utility of dev textures.