You're at far, far, far more risk for missing the years of professional development than you are for experiencing any signaling risk at negotiation, at least as of October 2015. Most non-profits have, by the standards of AmaGooFaceSoft, boring technical problems where 5 years of doing them is worth less than 2~4 weeks at their firms.

Though, that said: there's no guarantees that when you have the conversation in April 2019 that you'll be in the best hiring market for your interest in the history of ever, which is approximately where we are right now.