The speaker purchases in your home theater can usually be broken into four stages.
For most people, it is best to start with the LCR or left/right/center channels. The left and right channels are important because they handle most of the musical scores and dynamic sounds like gunshots and explosions. The center channel is equally important because it anchors the majority of dialogue in films.
When you are ready, you can purchase (a) subwoofer(s) or side surround speakers. If you have tower speakers as your front left and right channels, or monitors with respectable bass extension, then you can focus on your surrounds. If your LCR are not particularly adequate at lower frequencies, we definitely recommend one of our subwoofers. The 1961 Subwoofer 1V in particular punches well above its weight class.
The next purchases are usually the rear surrounds and/or two height speakers for Atmos duties. While these speakers don't need to be as dynamic as the LCR, it is important that they are timbre-matched for a cohesive sound.
Your final speaker purchases tend to be "the icing on the cake" like completing your height speaker layout, adding a second subwoofer, or even adding front wide speakers.