Yes you can. However, since tiers aren't official and aren't supported by Nintendo or their affiliates, you can't really press a button to play a UU battle in OR/AS for example. You can play tiered battles online in those games, but both players would have to agree with the rules. Nothing's stopping them from breaking the rules, since the game itself doesn't know the rule set.
If you do want to play Pokemon on a site where the software limits players to play by Smogon's tier rules, use Pokemon Showdown rather than X/Y or OR/AS. Showdown is better than in-game wifi battles anyway; it offers better opponents, faster battles and doesn't require you to train a team, just type it out.