You should always buy something because you never know if something unpredictable will happen, and you’ll be in some way prepared.
Also, the “save all your cash” move might not work too well, because you have a limited number of units you can buy on a turn. I know 20 for the east coast seems like a hell of alot, but when you’re talking ships AND planes AND transports AND infantry AND tanks/artillery, you might be short.