Attending a conference in a field of interest outside PhD field?
I think this is more a pragmatic matter than a question of whether it's appropriate or not. In a vacuum, yes it's entirely appropriate to attend a conference outside one's own specialty, perhaps even without presenting (just going to learn). The big question is: how do you fund your attendance?

It's quite likely that you plan on asking your advisor, group leader, or department to pay for the trip. In that case, would you going stop another group member from going? Perhaps a PhD candidate about to defend who'd want to present their research and look for a postdoc position? That might be considered stepping on toes, yes. You'd have to talk to said group members to find out how they'd feel about it. However, presumably whoever holds the money sack has the responsibility to divvy up the funds in a beneficial way, not you. So go ahead and ask if you can and should go, and see what happens. Perhaps there is enough money for all of you to go, in which case everyone should be happy with the situation.