Should I let my iPhone die before charging to save battery life?

NO. NEVER let your iPhone battery completely discharge. It will do the opposite of what you hope. It will degrade the health of your battery faster. Apple specifically recommends NOT letting the battery dip below 20%.

In addition to the link provided by Chris above, read this --> When to charge your iPhone or iPad - Apple Community

See this about optimized battery charging. One this to know though is not to completely discharge your battery before charging it. About Charge Limit and Optimized Battery Charging on iPhone - Apple Support also Batteries - Maximizing Performance - Apple

Should I let my phone completely die before charging to save battery life?

No.....this is about the worst thing that you can do if you are looking for good battery performance. Try to recharge the phone when it is down to 20% or so.

Don't do it.

I was doing it on almost all of my phones and on almost every single one it died within 2 years.