Yeah you can definitely get by without a flash or a reflector but you generally want to have the sun at your back when it's later in the day otherwise their faces are going to be underexposed in most cases. However by having the sun at your back you would lose that awesome rim lighting in the hair. So it's a bit of a tossup, you can easily add fill light in lightroom afterwards but you can't really add rim lighting like that. It's kind of a lame trade off, and the easiest work around is to just get a reflector or flash for the versatility.
One of my favorite photographers is the late Galen Rowell, if you haven't seen his stuff before it's definitely worth checking out. This quote of his changed the way i shoot forever. "I almost never set out to photograph a landscape, nor do I think of my camera as a means of recording a mountain or an animal unless I absolutely need a 'record shot'. My first thought is always of light." Think about that next time you shoot, you more or less shot your pictures in the magic hour which is some of the best lighting of the day, just think about taking advantage of the available ambient light and compose your shot after that. I would wager that a shot with a boring composition with exceptional lighting will turn more heads than a poorly lit shot with awesome composition. A nice setting is great but nice lighting is key.
Now once you delve into multiple flashes and reflectors you can more or less create and replicate any lighting setup you want wherever you want, but this is an art in itself. Once you train yourself to notice light sources and shadows and compose your shot from there you will see your work improve greatly. One thing i find myself doing a lot is looking at the strobist group on flickr and taking note of the pictures i find pleasing. Most of the people will post their lighting setups and you can make a mental note of what looks good and what doesn't, and if there is no lighting setup data i try to figure it out on my own. You can pretty much deconstruct any picture, i know quite a few of the strobist guys have gotten yelled at for cutting out pictures from their wives magazines and putting them into "inspiration" folders but i think it's a great idea.
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