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This will depend:

if you're going to add more packages to your "base ROS install", then keeping the build and devel folders around might reduce build times, as depending on what those new packages need, certain pkgs that are dependencies will need to be rebuilt.
if you're not going to add any packages (that would be a rare situation, but can happen), or are going to create an overlay workspace (recommended for source installs), then the build and devel (and their _isolated variants) can be removed.

Finally: a general tip for working with source installs: enable ccache. Even if you need to rebuild large parts of your base workspace, it'll be very fast, especially in cases where not a lot of files have been updated / changed.

This will depend:

if you're going to add more packages to your "base ROS install", then keeping the build and devel folders around might reduce build times, as depending on what those new packages need, certain pkgs that are dependencies will need to be rebuilt.
if you're not going to add any packages (that would be a rare situation, but can happen), or are going to create an overlay workspace (recommended for source installs), then the build and devel (and their _isolated variants) can be ~~removed.~~removed. As long as the overlay workspace extends the install space of the underlay, of course.