I guess the trickier question comes when someone has a lot more domain knowledge and some contacts in the field. This knowledge can translate into a "good idea" that is solving a problem in the industry. How much does this factor in?
If one persons contribution is low to point that an even split couldn't be justified, then I would just make sure that you really need them as a co-founder, as apposed to an early employee. If things are close though I would try to justify the even split so everyone is on the same plane.
In my opinion its just not worth getting greedy over small percentages, if it could compromise the success of the team or cause internal strife. The following quote comes to mind: "I'd rather have a slice of the watermelon than a whole grape."
If you are just starting out you might want to consider an equal 1/3 split between everyone. For our startup, I've been doing most of the work up until now, but I still insist that we each have equal share. I want everyone to be accountable and feel responsible for the companies success. Basically, no one should ever feel less obligated due to a lower ownership percentages; we all have to work our asses off 100% in order to win. Even small changes to the initial split could alter this balance. At least that is my opinion.