Of course, if it is reasonable.
But realize what the I-140 and GC process are about. It is about what you will do AFTER you get the green card. Your job description and conditions cannot stray too far from what the PERM/I-140 states, even if the GC is approved years later. Otherwise it is possible that I-140 would need to be modified.
Here's a possible abuse: your PERM was processed (job posting, etc) with a particular salary and job description, and no one was found willing/able to fill the position. If days after the PERM and I-140 are approved they hiked the salary for that job by $50,000, USCIS could see that as a fraudulent attempt to ensure PERM would pass.