My opinion, as stated elsewhere, is that you learn how to teach well FIRST. Not always possible for the young and the cashless, but trust me. It matters.
Although I started off with the same experience Samurai describes up above, that has changed. I teach with a JTE most of the time, but at least a few times a month, I teach entirely alone. I went to college to be an English teacher, and I know how to teach English to native speakers. That puts me in a slightly unusual category, and I know it. I went out of my way to learn Japanese words for grammar points, and I certainly do teach grammar when necessary. If the JTE is present, then the JTE translates, if s/he is not, I do it. What Samurai describes is a typical arrangement- my schools toss that right out the window when they learn I am an actual English teacher. Half the time, I end up teaching the JTEs about obscure etymologies or grammatical peculiarities. I'm a veritable encyclopedia of reasons for irregular verbs.
Also my schools have no issue with me speaking Japanese to my students. That just wouldn't make any sense. Not when I have classes where I teach alone. And even when I don't, I often go around and check individual students for understanding, and have them translate back into Japanese for me. I can usually catch misunderstandings that way. Especially in the case of nuance.
Oh yeah. This
is my "REAL" profession. I am so tired of people who think this isn't a job with real responsibilities. It is no less important than teaching in your home country. If you feel differently, then teaching is Not. For. You. You'll do your students a great disservice and they deserve better.