It is good to understand exactly what the classes do before adding them to your page. If at all you copy-and-paste, you can always use devtools to inspect the elements.
So it is not a must to memorize every class but the more you use bootstrap the more classes you remember off-heart
if you mean that you should know css and bootstrap i would say yes. i do the copy and paste things in css and modify them afterwards to meet my needs. i know the basics but i still struggle with the design more than with anything else.
so i would say it's better to learn styling and design instead of trying out a lot of methods until you randomly get what you want, since it's too time consuming