the choice of buttering one side or both sides of toast is entirely up to personal preference.
Traditionally, people often butter only one side of the toast, which is typically the top side, as that's the part that's more exposed and easier to spread butter on. However, some people might prefer to butter both sides, especially if they want an extra rich and buttery taste.
Ultimately, it depends on your taste and dietary preferences. Some people might prefer to use alternatives to butter like margarine or spreads, or skip butter altogether for health reasons. Feel free to experiment and find what you like best!