Who positions their speakers straight ahead (zero tow in) and why.

To many variables to consider on this subject.  Your ears must be the judge.  Sometimes, if your loudspeakers are too far apart - some small toe-in will help bring the center image into focus. This is a simple - no cost variable that we all face with box loudspeakers.

I have a pair of vintage Polk SDA 1C's, and I position them with zero toe-in because the manual recommends it. I think it's also proper given the interconnect cable for the extra stereo spread feature.

+1 @Locknow

+1 @Atmasphere

Equilateral triangle concept is great, however try this ratio:  distance between tweeters 1.2 times the distance from each tweeter to the ear in listening position.  I love big soundstage and getting the speakers as wide as possible (certain designs I.e. D’Appolito like my big Duntech Sovereigns lend themselves even better to this however I’ve found it fairly replicable across different designs).  
 

Ralph’s point about sidewall reflections also an excellent contribution- coming from a guy that has forgotten more about audio then most of us will ever learn :-)

Experimentation is key and often that’s the fun part!

Think of speakers as backwards microphones, which they are, you would always want to point the microphones exactly at the thing you want to record because the mic is designed to be most accurate on axis. Off axis coloration in microphones is very important and the thing that separates good and poor sounding mics same with speakers they sound most accurate exactly on axis. 
So
Point your speakers at your ears the left speaker at your left ear and the right speaker at your right ear. The off axis coloration of speakers is best dealt with by room acoustics. Thinking of speakers as reverse microphones makes many things in acoustics obvious. 

I ended up with my Magnepan LRS+ pointed straight ahead after trying many other positions..