It's good that you are eager to help in any way you can, but as a tank I'd prefer to be above half if possible at all times, not because we're at any risk of dying but simply because we don't like to be that low (same thing with losing hate, it's not a big deal if the dps can tank it but it still makes us feel like we failed at our task haha).

Personal preference is to be full as much as possible without overhealing, but a lot of WHM's aren't good so if I'm at half with a bad WHM things could go bad quickly.