I posted pretty much the same question as you a couple of weeks ago for our product http://flashissue.com. at the end of the day it's important to just measure what's important in YOUR funnel.

it's taken us a while to get mixpanel installed so we can measure this properly.

our key metric is not how many people we get logging in but how many people move through our product and send out an email newsletter. we'll be a/b testing this with a "sign-up" button and an "open" app.

we should compare findings :-)