I think you should at least use bootstrap reset and grid system, as by doing that you're following conventions, that's been followed all around the world, so it's very easy for any other developer to take your work and pick up from where you left off. not to mention the upgrades, for example, we were using bootstrap 3, and it's using float, now bootstrap 4 is using flex, so we just upgraded the library, and adjusted some classes, and in half an hour we converted our whole grid system.
so, to wrap up, of course you can write your own grid system in 2-3 minified css code, but then the other person taking your work will have to understand those unnecessarily customized code, and that code you'll have to maintain it through the life of the app, but by using Boostrap grid system, you're using the standard.