My $.02 says not to fill. Hardwood, as you know, by nature expands and contracts. And depending on conditions in the house, above AND below the floor, you want that expansion AND contraction to happen. (Again, depending on the conditions in the home.)
It's probably safe to say that if you do fill the gaps 'at the wrong' time (like when the boards have contracted), they will buckle on you when they expand. When new flooring is installed in the home, installation instructions usually call for 'constant' conditions to be in effect at ALL times during the year. In fact, some board makers ask that the flooring be allowed to 'settle' for weeks before it's installed so it can become 'aclimated' to the home's temporal and moisture conditions. This way, the flooring boards can be at a 'normal', home setting and be butt-up against each other when installed. And whatever little expansion (or contraction) there is, the flooring can tolerate it. If conditions vary enough in the home, filling the 'gaps' can be detrimental.
That's all I can offer. My best to ya and hope this helps.