There are mixed responses to this problem. First, if your floor is a traditional, 2-3/4" type of hardwood floor, I wouldn't fill the gaps. Second, if the boards are, say, tavern-style boards, it may be OK to fill the gaps. I don't think, in short, that the gaps should be filled. BUT, others may differ. I wouldn't warrant the work, that's for sure, unless you took proper measures to fix the CAUSE of the expansion and contraction, ie., humidity.
The more humidity there is, the more the wood will expand. The less humidity there is, the more the wood will contract. Once you've 'stabilized' the humidity in the house, the wood won't move that much, if at all. You need to consider adding a humidifier to your house if you can. If you have forced, hot air, it's easy to do. Otherwise, the 'solution' may still lead you to divorce!
Here are a few past posts on the problem of filling gaps. In addition to what others say here, formulate your own response to your problem. My best to ya and hope this helps.