If you have luggage like panniers or a rack or handlebar bag then everything is likely to fall out. The bottles may fall out if you forget to remove them. And it would make removing and replacing the wheels more difficult as you don't have gravity to help you on removing, nor can you use the weight of the bike + your weight to seat them properly on replacing. And it would be difficult reach the brake to give the tyre that extra shove/squeeze to get it past the brake blocks if you have caliper brakes and limited clearance even with the brake opened.
I just keep the panniers closed.
The big basket on top of my rear rack contains a backpack, some bags, and a mountain backframe on top, with some plastic sheets against rain above it, held in place with 4 flexible cords with hooks. The frame serves as top closing lid.
Sometimes when having bigger luggage I just attach it above it, this is a picture of such case, you can see the plastic sheets and the rear of the red backframe:
So you can imagine (without the rolls fabric ofc) that when I flip the bike upside down, it sits very stable on the ground - the height of the metal cage basket + frame is same as saddle.
About gravity, a bicycle wheel weight is not really a problem, no? Much easier to handle than entire bike.
And reaching the brakes - the panniers tilt / fall till the ground, they rest on the ground, leaving the entire wheel sides clear including mudguards.
Mine is a quite heavy bike. But alot weight also on top (handlebars bag), basket+content on rear rack, so the weight is vertically quite balanced, making flipping the bike easy.