Yes. That's it. We've been doing that with GSM phones for years. Now that Verizon and Sprint have joined the party, it really is that simple with an unlocked phone. However, see the caveat below in case you ever need to get a new SIM card (I've heard Verizon is better about replacing SIMs for existing customers).
The only reason you might do that is if you are thinking of switching to Verizon or Sprint (or a regional carrier with a legacy CDMA network). Since the SIM-Free is supposed to show up on their white lists, it will be less of a hassle trying to get them to give you a SIM card. My parents could not get Sprint to give them an iPhone 6S SIM card for a "T-Mobile" iPhone 6S (even though they already had a Sprint iPhone 5, Sprint's SIM card changed). They then tried to port over to Verizon, who gave them the same runaround. Fortunately an Apple Store had the Verizon model in stock and just did a swap. If they had the SIM-Free back then, this (theoretically) would have been avoided.
Thanks. I'm 99% sure this is the route I'll go.