As long as the seal works for stowing, it's safe enough.
As for appearances, well a new one would look better. You might take the old one with you to the dealer, I'd ask them to replace, and see what you can negotiate.
However, you can sort of tell, just by hefting the tank. When you can easily do it with your little finger, it's time for more fuel
. If you're used to hefting sacks of flour, when it's down to 5#, buy more. If you're going on a long dinghy
explore (over 10 mi.), buy more, top it up. It's generally a good idea to have at least 4 liters spare on the mother boat