Theme a admin block without specify Block Id is better practice because local block id and server block id may be differ in that case we need to update our code. In some time we need same theming for multiple block, We add like #block-22,#block-23 in our css file. If admin create that kind of block we again update the css file.
Avoiding this kind of issues we can use a module Block Class. If we use this module while creating a block user can add classes to any block through the block's configuration interface.;