Earlier quoted context omitted.
You can do whatever you want. If your FMV is much higher than the strike, the bigger problem is the tax bill.
That's why it makes sense if you're at the seed round and the FMV is still peanuts. Post-A round makes it much harder.
You have employees who own shares but need a nontrivial amount of cash to cover taxes. You have investors who may be interested in acquiring more shares (or the company may want to buy them itself). You have a notion of a current valuation of the shares. Broker a sale at a price that's beneficial to both parties.
What is the company's incentive to block such a transaction?
I'd love to see this become standard.