It is up to you and how much data you use on both devices. You are already paying extra for the tethering option so as long as that data limit is sufficient for both devices, I would forego cellular on the iPad.
I went with an iPad wifi-only last year and have no regrets and I don't have tethering on my iPhone (I'm not willing to give up unlimited data on it and pay more for tethering). The times I've thought cellular would have been nice with the iPad have been very, very few, and even then, not truly necessary as I could do what I needed to with just the iPhone by itself in those pinches. I've read consumer reports that mention that the majority of people who buy cellular iPads never activate service anyway, and wifi hotspots are just becoming even more numerous and common, not less so.
One big caveat would be if you want standalone navigation/location services on your iPad, as the wifi-only models do not have a GPS system (they rely wholly on wifi-node ID for location services). Although for about the difference in price between wifi-only and wifi+cellular iPads, you can add an external (BT or connected) GPS receiver.