In the past few years, power supplies with top fans have kind of taken over the market and replaced the traditional rear fan power supply and there's been a lot of concern over whether it's safe to replace a rear fan unit with a top fan unit; in short the answer is: yes, it's perfectly safe.
A lot of the concern comes from confusion about airflow within power supplies. With rear fan power supplies there's venting on the side opposite of the fan. Air flows in through the venting and is then exhausted out by the fan.
On the other hand, with top fan power supplies there's venting on the rear of the PSU. Air is pulled into the power supply by the fan and then flows out through the venting.
By now you might be asking, "So if the air is moving essentially the same with both styles, why the change?" Well, there are a couple reasons:
1. Top fan power supplies have bigger fans.
RPM for RPM, a bigger fan will generally be able to move more air than a smaller fan. This means that a top fan power supply can run its fan at a lower rpm than a rear fan power supply and produce the same level of cooling, as a result it makes far less noise.
2. Depending on your case you can separate a top fan power supply's airflow from the rest of the case.
Many cases now include vents for top fan PSUs. These vents enable the power supply to draw in cool air from outside of the case rather than warm air from the case interior. Since the PSU is not taking in warm air its fan won't have to work as hard and can run at a lower RPM which once again helps with noise reduction. The PSU's longevity is also benefited by the cool air - the life span of electronic components is often a function of heat, so the cooler things stay the longer your unit will keep running.
If your case has a vent for your top fan PSU we recommend mounting it so that the fan is pointed towards the vent, otherwise we recommend mounting the PSU so that the fan is pointed towards the interior of your case. For more details and reasoning please see the question above.