locking tables in database, is it needed?

[eluser]Unknown[/eluser]
I'm sorry for my question, others more familiar with databases probably find it pretty trivial:

I have a many-to-many relationship (products, categories, productcategories) and when I update a specific product (flushing and adding new categories in the connector table as well) I think I really ought to do LOCK TABLES (which I indeed do, using LOCK TABLES with MySQL), but there's no such method in CI's ActiveRecord class - and transactions are not supported with MyIsam tables.

I have read most tutorials I could find in Google on locking tables and I still think I have to lock tables, though I might be wrong: what's the proper way to do it? I would like to be database agnostic in case I end up with a Postgres DB, which does have locking, but clearly states that it is not part of the SQL standard... any suggestions? Thanks!