Here is another random tip:
Some climbing gyms have installed cracks. Mostly hand-cracks for some reason. You will of course bring fairly tight banana shaped shoes to the gym. If someone randomly ask you to demonstrate how to climb these cracks and you do not want to pay a few euros to rent oversized flat rental shoes, just pop the heel off your perfomance slipper so you don't have to break the feet to slowly climb some 5.9 hands while showing the proper hand-foot pattern.
I cannot believe how long time it took me to figure that out.
[edit] Popping off the heel can also be helpful when smearing on volumes on slabs [/edit]
I'll go out on a limb here and say IMO if your shoes are stiffly downturned enough to not work well in a crack then they're too stiff for indoors
