Im installing a amp and sub into my car, which has a factory radio. I always get a different answer from person to person when it comes to this question.
The simple answer is no, you don’t have to. The better answer is YES, it is preferable to sum both channels to one so that you don’t miss out on differences in recorded material from either channel. By taking signal from both R/L your amp then sums them together (MONO) and you get everything intended from the program material.
The correct answer is yes, you want both left and right so you get low end information from both channels. It is very common for recording engineers to use panning to create a soundstage which results in information in left or right channels. Choosing one means you will miss out on the information in the other channel. You also get the bonus of summing two signals which is free signal gain. Always, without exception*, use both left and right channels to feed a sub amp.
*some may say that there can be noise or cancellation issues when using two channels in rare instances but that is from improper install/wiring or damaged equipment and something that should be fixed with proper wiring methods, instead of a band-aid of using one of the two channels.