I held ISO stock on which I paid AMT in a prior year. Do I still need to determine my AMT even if I do not sell the stock?

Once you've paid AMT for your ISOs, you should always calculate it in future years to recover the taxes you essentially prepaid. The credit is not linked to your sale of the AMT-generating ISO stock. (See other FAQs for a discussion of the AMT credit and the AMT consequences of selling ISO shares.)