In my Proto-Traveller Homebrew Universe, I've kept things pretty cash-oriented on worlds with C-ports or worse.
Worlds with A or B ports IMTU offer a little more flexibility, at least for smaller transactions: If Freen Greely, Interstellar Adventurer, has had an account with the Zebulon 3 branch of Galactibank for more than a few weeks, Galactibank will have shared that information with its other branches within the subsector and beyond. If Mister Greely has remained in good standing, and if he hasn't been moving faster than the bank's couriers, he'll almost certainly be able to access modest sums (say, up to 10% of his account) from any branch without much trouble. If he's accessing millions, well, he's going to have to wait for an advance transfer of the funds to take place, and that'll take however many weeks it'll take for the couriers to get from where he is to Zebulon 3 and back. Of course, if he knows he'll be heading to the shipyards of New Bristol to buy that new yacht he's been itching for, he'll have the bank work the transfer in advance, and the local branch will probably have his money there before he arrives.
Planets off the beaten track aren't going to be in the loop, though. If Freen wants to go into the hinterlands to buy whatever it is he's buying in the hinterlands, he's going to have to bring cash.
Starships IMTU won't need to carry lots of cash if they're merchants doing milk runs between a handful of civilized (read, "A or B port") systems. If they're regulars in port, they'll have local accounts established, and probably will have good relationships with their bankers in each port of call.
On the other hand, tramp Free Traders dicing with death on with unrefined fuel on backwater worlds won't have the benefit of these networks - especially if they aren't working a route, but plunging further off into the boonies. These guys need to keep cash on hand for everything, because they can't wait for their credit checks to go through. Cargo needs buying, crews need paying.