Next to pianists the most frequent ensembles I get at the weekly chamber series I record are string quartets, have two to do this week and next. My setup is generally governed by the space and where the players choose to locate on the stage. The hall is a geodesic dome with stadium seating used mostly as a lecture hall, center aisle the main stage approach, very good sound but brighter when empty than when the audience files in. The instrumentalist area comes right up to the front row of seats, where I set up. I can't pull the mic stand too far back or I'll block the aisle, so when the quartet decides to sit forward I have to set up pretty close to right in front of them. This makes the ratio of the distance to the second violin and cello versus the viola and first violin more extreme than when I can step back a bit, and creates a bit of a balance problem. I generally do two separate stereo recordings, when I have time for setup. One is omnis on a Jecklin disk (mostly Josephson C617s with LD Gefell caps), the other Blumlein (AKG C426). Both of these produce very nice recordings, but friends with highend audio playback almost invariably prefer the Blumlein. On good headphones I would say this is also the case for the stereo image, but the omni sound is more accurate ("clinical" is the description if you want to diss it). I would hesitate to try the Blumlein in a room with poor acoustics, though, for reasons others have given. If what you hear in the room is tolerable, the Jecklin/omni route will reproduce it well, otherwise cardioids in ORTF or DIN (but I think you'll have to be careful in placing and aiming them to avoid balance problems with the closer violin and viola versus 2nd violin and cello).
Jeff