In general the wider the radius/diameter of curves the more realistic it looks, but of course that is where compromise comes in (and I am going through this myself, my wife claims me trying to design a layout is worse than a woman shopping for a wedding dress....). An O72 radius circle would take around 16 pieces of track (it depends on the brand), and would be anywhere from 72 inches across to maybe 73", depending on how they measure the radius. In general scale size equipment takes wider radius curves, the big steam engines, 20+ inch passenger cars and scale size freight cars. Some engines will run on smaller size rail, but they may not look all that great. Traditional post war O equipment was designed to run on relatively small radius/diameter curves with some exceptions, so if you are running that you may not need 072 level.
It comes down to what you are planning to run. While anything looks better IMO in wider radius curves, if you are planning to run semi scale or smaller scale units, you may not need O72. It also depends on your space, on my layout my main loops of track are O72 and O64 at the moment, because I have at least one scale sized engine (an older Williams brass Hudson) that pretty much needs the wider radius, it means though having less room for other features I would like to have and also meant, because I didn't want to have a table with removable access panels, that I don't have the ability to do a reversing loop.
Even if you are running semi scale if you have the room I recommend going with diameter above O36 for main loops and try to use as large as can fit and meet your needs. I used small diameter curves in some industrial areas/dock scene on my layout, because I love small units like docksiders and plymouth diesels working that kind of trackage, while my mains are larger radius for the through trains.