If it supports 802.11a then yes, it would connect at 5GHz to a compatible WiFi network. It's worth pointing out that the access point would also need to support 802.11a for the phone to be able to use that standard.
There's always a load of misinformation around any benefits a 5GHz connection has over a 2.4GHz connection. Yes it can allow faster data transfer but, 5GHz is also less able to penetrate obstacles, like walls etc. And unless you have mega fast broadband and a load of other users on the network, 5GHz speeds aren't necessary.
5GHz networks would have been less congested than 2.4GHz networks, when the former was in its infancy. However, as more and more devices support the band, more and more people will use it and thus congestion increases. I'd wager the vast majority of 5GHz users don't change the channel used by the access point(s) either, further exacerbating the issue.
To some up, let the phone decide which network it connects to and you'll likely get the best experience.
Paul