Well there's probably 100 different answers to that question because it involves many things. While Sock was designing his map Freeport Docks, we had some discussions about this topic.
I think that designing web graphics and maps is a different approach although some concepts do apply. It's also a very subjective thing and what I'm about to say reflects more my personal opinion and they are definitely not "objective guidelines" because I think there just aren't any for this aspect of map design.
While the the minimalist aproach might have been viable in the Quake map design "era", when Quake2 came out and then more recent games like SiN and HL, this is less and less practical and conflicts more and more with the game's new technological improvements.
The example that comes to my mind is Quake2: it was the first FPS game with colored light but you needed a 3D card to see it. Some people then recommended to design a map with monochrome light in mind. But then this causes some serious technical issues. It's virtually impossible to design a map that will both be lighted right with and without color because different colors at the same light intensity value don't produce the same resulting amount of visual brightness.
So what happened?... All the good mappers made maps with colored lighting in mind. What if the lighting (or the absence thereof) doesn't look good in SW mode?... tough luck.
Why? Because colored lighting, on top of being one of Q2's most important features was an integral part of it's maps "distinct personality".
And at the time, 3D cards were more expensive than nowadays (I remember paying around $250 for a Voodoo1 card back then) but that didn't stop designers. So just imagine today when most serious gamers have a 3D card (Voodoo2's can be had for dirt cheap). Many mobo manufacturers have started integrating AGP 3D video on the MB itself (altho I don't recommend buying such MB designs myself).
So now, here we are with newer and better games with a LOT more incredible features with even higher hardware 3D only games like Q3 Arena and FAKK2 not too far away. And we designers should sacrifice all the cool new features and design for the guy who hasn't upgraded his PC in 4 years in mind?...
Sorry but no way! Besides, have you ever looked at SiN in SW mode?... Man, it's so ugly it looks like it's not even the same game. Personally, I think that making good maps (especially SP) is complicated enough as it is (and it will get more complicated in the future) without us having to worry about 4 year old technology.
BUT... I have to ponder this by saying that one should always test his maps on slower machines as well but you have to draw a line somewhere. Here again, this is entirely subjective but I think that with the low price of those Intel Celery's, the absolute lower limit should be a P200 MMX with 64Mb ram and the "average" lower limit should be a Celeron 300 with 64Mb both with a 3D card.
The bottom line is: by nature, we always have to make (sometimes big) compromises when designing maps but to design with SW mode in mind at this point in time in pushing the idea a little too far and a thing of the past: the argument that 3D cards are too expensive no longer stands. If you want to play todays's 3D games, you need a fast machine and a 3D card period.
My 2 cents
The official Sin entities and scripting reference site