I always use speakerphone whenever I can. Interesting side note here: I started using speakerphone before my tinnitus increased (before tinnitus bothered me), because I had this bizarre symptom where my brain would vibrate/flutter. Very weird sensation, and I'm not sure if it was doing something to my vestibular system or something else. It stopped doing it but I've changed phones many times since, so I'm wondering if it was something to do with that phone in particular.
All I can say is that it felt seriously strange. In terms of tinnitus, I don't think it has an effect, but the studies I've read on the effects of mobile usuage just seem inclusive. I don't think anyone knows for sure what happens to our brain when we have a phone pressed against it all the time. There was a time when they linked brain tumours to mobile use, and then they refuted it. Who knows?