Alternating between 5s as a cellphone and 5s as an iPod touch

I bought my wife a T-Mobile unlocked iPhone 5s for Christmas. Before we decide on which carrier to use (T-Mobile or AT&T), we'd like to just start out using it as an iPod Touch. We do plan to activate it with a cellular plan (month to month), just not immediately.

If I understand the process, we'll first need to leave the SIM in and activate it with T-Mobile, but without actually signing up for a service plan. If we decide to go with AT&T, we'd first need to get and install an AT&T SIM. Then we'd activate the phone with AT&T, again not subscribing to a cellular plan until we're actually ready to use the 5s as a phone. Is this correct?

I'm curious about all this because there'll be times when we won't be using the phone for a month or two at a time. In these cases, we'd like to be able to still use the 5s as an iPod, without paying a carrier, just by allowing the month-to-month plan to lapse.