Db schema migrations in MySQL
There is obviously no rule, but it's probably a good idea to be consistent. It's also a question of whether these schema changes will be executed by someone perhaps less skilled in running SQL scripts, e.g. if the database and its application is a product you're distributing to others.
While MariaDB supports statements like ALTER TABLE my_table DROP COLUMN IF EXISTS col_name; (see here) , MySQL doesn't have this. You could create a stored procedure to do this, perhaps something like this.
Stored procedures are great for a number of tasks, but this sounds like more work than it's worth. I won't reject the idea completely, as it all depends on your specific use-cases. When migrating big tables you may instead want to use a tool like pt-online-schema-change in Percona Toolkit.