Sound advice, but I never seem to be able to stick to it.
If I pay no attention to aesthetics, either I get bored halfway through playtesting because there's no atmosphere and so I'm not feeling it; or I complete the map, look at the amount of work it'd take to detail the entire thing, shrug and leave it as is.
So I end up starting from scratch; thinking this time, I'll just get a nice little theme going in the first room, with minimal detail, and stick to that with no additional effort. Yet inevitably, a few hours later I find myself toying endlessly with wall detail.
The latter still works better for me, because, well, that's the only way I get maps done. So if I can't change my global approach, second best option is to improve my detail process.
I try to stick to the most efficient parts of my previous maps - efficiency as in best ratio of "does what I want it to do" over "how much time it takes to build". 90° angles, 45° angles may not be the most popular thing, but personally I've noticed the time I spent doing curvy things being completely unremarkable ingame more often than I was bothered by a straight angle (and there's a lot more straight angles in all PWADs than there are Phml-made curvy things around), and sticking to simple angles make building, moving things around and rebuilding easy and fast.
I try to iterate fast, and I'm trying to get the habit of keeping every iteration of a particular piece of detail around until I'm completely sure I'm finished, so I can compare as I go. There's so many little things I discover every day that makes me map more efficiently, too; like building rooms in a vacuum and then deleting parts of a working but currently bare sector while keeping a strategically placed vertex so I can paste the aforementioned room into the base sector and join everything neatly, with no need for tedious texture rework or the inevitable oddities with overlapping linedefs, despite merging sectors. Also keeping a copy of said rooms in their own little space so I can make changes quickly, instead of having to deal in the playing area directly.
Perhaps in a way it's a manual, inefficient route to mimick prefab systems; but I find forming the mental patterns to quickly draw various doors, pillars, stairs and structures help me build faster as well as allow for more flexibility in my thought process, because I start to get an intuitive feel for the shapes. Lately I find I can design entire room visuals in my head down to line lengths and everything and have a pretty good idea of what it's going to look like, how it's going to all fit (perhaps this sounds trivial to those of you with artistic skill, but it's rather new for me).
Another benefit to caring about aesthetics as I go, to me, is while working in autopilot doing detail busywork my mind is free to wander and think about gameplay scenarios further down the line, or how the visuals can integrate with the action. There's only so much creativity you can put into something until you run out, so having those peaks and valleys in my mapping is actually beneficial, I think.