Is it necessary to perform a front end alignment when installing new struts?

Absolutely.

Be aware that compressing coil springs is extremely dangerous work. The toy spring compressor tools you can buy for 30 bucks makes the job even more dangerous. It's not that expensive to have a mechanic do the job correctly, then align the car right away. They will also inspect the upper mounts.

A better alternative is to buy the assemblies with new springs and upper mounts already installed, then you just pop the assemblies in place. There's little danger to you but you'll be paying for parts you may not need. Your mechanic may still need to take the lower parts apart to make the alignment adjustable so he could be redoing part of the work you did yourself.