It shouldn't do a lot either way - as far as I'm aware. As long as the components of the PSU stay within their temperature limits, it'll be fine. The only thing you could run into is that the PSU's fan pulls air in and blows it out the back. The fans you say are normally where we're looking through sound like they're blowing the other way which would mean some air being drawn in through the PSU - the opposite way to its own fan. That would be fine and would keep it cool but if it gets hot enough to need to switch its own fan on, it would then be fighting against the case fan and could, perversely, result in less cooling if they cancel each other out.
You could either reverse the case fans - although that doesn't sound massively practical - or the PSU fan. If you do reverse the PSU fan, it would invalidate the warranty (by opening the box) and also, you'd need to make sure that you have drained the capacitors (power it on with no mains attached and then wait a while) before you go poking around - otherwise it could still have some juice left in it.