Best practice on importing Craft db backups?
The short answer is, yes, definitely drop your existing tables before importing a new .sql file. If you don't, you run a high risk of Craft developing weird issues of data mismatching. This is especially true if the two versions of the database were not created with the same exact version number of Craft.

NOTE: You may want to back up your existing database before dropping all of the tables.

The longer answer is, database syncing is an interesting and complicated problem, with many possible solutions. Check out this thread discussing the "holy grail" of database syncing across multiple environments...

What&#39;s the best practice for handling data migration and organization across development environments?

Ultimately, your data migration patterns are up to you. There are varying approaches, some involving Shell scripts and Git hooks. It's an evolving challenge for the Craft community (and other CMS's as well), so keep your eyes peeled for the next brilliant "eureka" moment!