how necessary is it to have a bottom mounted psu in a crowded gaming case?
to be honest, a bottom mounted PSU is a good idea, it will allow the rising heat to be caught up in your systems cooling system (Normally the rear exhaust) keeping the airflow directional. having a PSU at the top of the case with it's vent pointing downwards will cause a small amount of heat to get caught at the top of the case increase the temp. (i also think the the bottom mounted PSU will allow you to have a top intake fan too in most cases)
Often a bottom mounted psu can require extensions to reach all necessary drives or mobo connectors etc, but the weight of it at the bottom makes the unit more stable, I wouldn't say theres any particular benefit/drawback performance-wise to either though.
Luca posted at same time as me, but I'd add that if your airflow is good anyway, You'll have few heat problems
I don't think that it has that much of an impact, I'd look for a good quality PSU and case I like, and if they're made to have the PSU at the bottom then I'd roll with that.
If the ''top of the case'' PSU has an intake fan on top and holes at the back of the case then it's airflow would be pretty much identical to a bottom one.
I had a brief thought once, Heat travels upwards right? so the bottom PSU leaks some heat upwards (towards the GPU/MB) but a top PSU would leak heat outside the case right? I don't think it's enough heat to make a difference but it's still a thing to consider.