I think it kind of depends. Is it just that you're not interested at first but know that once you get going you'll become interested? Or are you completely disinterested in the whole thing? When I was on Depo Provera it was a few months after my husband and I started dating and were living together. The Depo shot severely cut my libido and I was never really interested in sex unless he started. Then, once he would start I'd get interested. I did not consider that faking, but I also was open with him and let him know how I was feeling and so we were open about it with each other. Once we knew it was the Depo we just got rid of that ASAP.
I think it's a case by case basis and I don't think there's harm in it as long as you're at least somewhat interested, but if you're not at all and you're not letting him know this it could be damaging in the long run if he ever finds out you weren't interested and also if you're not interested often maybe there's something you two could do to allow you to be more interested.