Very mixed...
I had and have situations where money pures in so fast that you need to think of ways to spend it cleverly. A cash reserve for emergency situations or to take opportunities (though events are technically gentle enough to let you sell stuff while they are still lingering) is a good idea, but piling beyond that and becoming Dagobert Duck probably not. OW's global market is a nice tool to balance money and the physical ressources, but art starts when you think of ways how to transform those into training points, oders, city capital or research. Beside many workers improving the land around the cities and looking out for governors who can rush specialists for money, I think a good use for money is using it to influence other characters or nations.
But thats only one side of the medal - you can be short on money, too. I'm experiencing this currently in the 3rd mission of the Aegean Campaign...my deficit is around -140 per turn and I'm forced to sell ressources. I should have invested in Hamlets early probably, but I decided to leave my usual builder appraoch and refused to sign peace with Thracia, instead going full conquest on them. Being cured by a few past games I had to abandon because of the AI simply outnumbering me, I built some more units - with the result that I clearly won the war, but now struggle to keep my economy together.
BTW since I'm now the first time close to the situation...how does OW handles debt/bankruptcy? I already noticed that probably some kind of auto-sell/purchase mechanic is at work, if one forgets to have enough in store to cope with a negative turn bilance for a single ressource...but what happens if too many ressources have a deficit and nothing is there to be sold to compensate over the world market? I haven't found an answer in the help and I'm really curious, as it is usually a situation most startegy games fail to bring up a resonable, fun and none-exploitable solution for...but I could imagine that OW has one, given how well designed the economic system is overall in the game.