Pewter is reasonably maleable, true. However, the compact shape of dice would reduce any distortion caused by rolling.
Even pewter character figures can take quite a bit of falling over before they show signs of distortion (bent swords, scraped noses), and they are much thinner metal and less regular shape than any die.
Rolled on a softish surface, like a tablecloth or a dice-mat, I'm sure the pewter dice would stand up to regular use. After all, cups and plates were made from pewter in medieval times (until they realized about the lead!) and were preferred to clay because they lasted so well.
BTW most pewter is lead free nowadays. It might be a touch harder in the new formulation, I don't really know... any metalurgists out there??