I have a Lancool Dragon Lord case for my new system. I've also had three other cases with bottom mounted cases. You have two choices with the a bottom mounted power supply.
First, if your case has sufficient ventilation, airflow, and cooling, then you could mount the psu fan side down so it draws in cool air from the opening in the bottom of the case and exhausts warm air out the rear of the case. The idea is that by drawing in its own supply of cool air rather than drawing in warm case air the power supply will be easier to cool and will last longer. However, do not place the case on a carpeted floor, especially deep pile carpeting. The carper fibers will block air flow. A few years ago I made my own pc stand with casters.
Second, you could install the power supply with the psu fan on top. The concept is that the psu would help cool the interior of the case by drawing in warm interior air and exhausting it out the rear of the case. This concept goes back many many years when Intel started making cpu's that ran hot. The cpu's did not have cpu heatsinks or fans. In addition pc cases did not have case fans. In the original atx standards Intel specified that psu's should be placed near the cpu so the psu fan could help cool the cpu. Obviously this standard no longer applies.
The choice is yours to make based on your own specific ventilation, airflow, and cooling situation.
The ventilation, airflow, and cooling in my new Dragon Lord case is excellent so I mounted my power supply with the psu fan on the bottom.