I dunno, the humidity where I used to grow my Neps (windowsill style) was really bad... sometimes no more than 30%. I never had any problems getting mine to pitcher or grow. I say experiment with a few expendable Neps first and see how they grow in the conditions you already have...... but that's just me.
EDIT: In fact, when it's not raining in the winter, the humidity outside is really low, too. All of my Neps pitcher fine outside, whether the humidity was 30% or 60% due to rain out in full sun and "tough" conditions.