I was reading an article about Saisons in Zymurgy a while back. Drew talked about how yeast don’t like pressure and to NOT put an airlock in but rather just cover the opening with some sanitized foil. Now I do this for starters with the theory that there will be better exchange of gases. Here is where I am baffled…how much back pressure could putting in an airlock create? It can’t possibly be more than .5 psi. Could it make a difference?
Even if it’s +1/2 psi, that’s still 50% more pressure than 1 psi. I thought it was higher, but I don’t remember where I saw that.
EDIT: Whoops, I’m an idiot. I meant 1 bar, not 1 psi. 1 bar is like 14 psi, so 1/2 psi isn’t much pressure at all.
Another reason why I don’t think it is a big deal is that atmospheric pressure will vary according to altitude. In San Diego it is essentially 14.7 psi but in Denver it is something like 12.2. Obviously brewers in San Diego in make great beer even though their yeast have to endure an extra 2.5 psi of pressure compared to Colorado brewers. I think this myth is busted… 
I always brew my beer in the troposphere. The yeast love the near zero-gravity environment.
1 psi = 2.3 ft of water
Height of water in a airlock is ~1". =>back pressure is miniscule.
Using foil on starter may be beneficial because it allows diffusion of air into the starter vessel.