If you have multiple headshells, you only need to align the cartridge once for each specific headshell using an alignment protractor. Once the cartridge is perfectly bolted into position on the headshell, it stays aligned.
However, because different cartridges have different weights and heights, every time you swap headshells, you must re-balance your tonearm, reset your Tracking Force (VTF), and potentially adjust your Tonearm Height (VTA) before dropping the needle.