Should I amend my 2018 taxes if I neglected to enter the full year of medical premiums that I paid while itemizing deductions?

It depends on the total amount of medical expenses that you are currently claiming. You only get to deduct the portion of your 2018 medical expenses that exceed 7.5% of your Adjusted Gross Income (AGI). If your current medical expenses are over or close to being over the 7.5% threshold then you will benefit by including these expenses.