The definitive answer to your question actually hinges on "how big is the leak?".
If the leak is so slight that it requires a few days for the gasoline to drown the engine and contaminate the crankcase, then you should be OK and take comfort in knowing that the inline valve will stop the issue.
But, if the leak is so great that your fuel/air ratio becomes fuel rich (black smoke out exhaust and fouling plugs with carbon) then you should go ahead and fix the problem.
If a carb kit was installed including a new needle valve and the valve seat was replaced, I would have to suspect that maybe the carb float may be defective. That too will cause a carb to drown an engine.
If, you can remember to always shut off the inline valve before storing the tractor, you should be fine.
Do whatever is necessary to help you remember (if you think you might forget). A note on the dash or a sign in the storing location.
Since you know your carb still has an issue, I suggest that you turn off the inline valve while the engine is still running, and let it run some gas out of the carb before turning off the key switch.

Great advice, thank you.

Can't the carb be rebuilt for a fraction of the cost of a new one?

OP stated in the beginning that a carb kit had been put in.

Guess a carb kit and a rebuilt is the same, But if it was rebuilt you would think that it has a new float put on also, At least that's what I would have done, You can always take the float off and shake it, If you hear something rattling inside then it is bad, Just seems funny if it was rebuilt that it is still leaking gas somewhere.

People have different definitions of "rebuilt".
To me, just putting in a kit is a "tune up".

A "rebuild" would be a deep soak in "real" carb cleaner, all passages cleaned out, any sloppy bushings rebushed etc.

With the ethanol in gasoline being a "water magnet", many internal passages may get so corroded, they can't be properly cleaned out. It just depends on the degree of abuse.

Often with high labor rates, it's better to get a new carb then pay someone to do their version of a "rebuild" that may not fix the problem.

Thanks to all. I think the "rebuild" was a carb kit and I think the float was replaced too. The dealer said he's seen this problem commonly with the Kohler magnum engine of this vintage and sometimes the only thing they can do shy of replacing the carb is to install the shutoff valve.I'll give it a try. When I was in college, I worked summers for the Kansas Turnpike Authority and we always shut off the fuel lines to the tractors at the end of the day. Maybe I can get into that mode again. I actually don't mind spending the money on a new carb if I was sure the rest of the mechanicals would last long enough to make it realistic. I've also started to look at other options. A five series JD (the 500) looks really good but may be more than I need. I was really thinking maybe a 320 with a 54" deck would be adequate. The axles look pretty sturdy. I mow about 3 acres but not much heavy duty work. I use the Gravely snowblower mostly to knock back the snow so the pickup plows have someplace to push it. I could do that with a walk behind near the house. (Our driveway is three tenths of a mile long). Any opinions on a setup like that? How hard is it to use the foot operated lift? The Gravely's lift is hydraulic which comes in handy to adjust the deck height over our uneven terrain on the fly. Would I be able to do that with the non hydraulic lift? Sorry for the wordiness. And thanks again.

***"People have different definitions of "rebuilt"."***
I guess that is right. I personally do not ever invest in a kit without a thorough clean out, probing and compressed air.
And wear in the shaft bushings gets checked before buying a kit.
So I may have been reading too much into a statement regarding work done by another person.