Depends on how far you drive from home as to how much cracking is ok.
I have had great luck running very cracked looking tires. As long as none of the cracks look really bad and they are all small "age cracks" just air them up to about the max listed on the tire and use them.

By running them with a higher pressure they flex less and the cracks don't get bigger.
If they flex much they will fail sooner.
But that's just a guess, as I have never had a flat on my truck with some really old 7.5-17 and some reasonably old 8-19.5 tires, about 8 years now, and 10 or 15 thousand thousand miles.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-