I have noted that the default SharePoint databases are created with a "Full" in Recovery Model, 1 Mb autogrowth on Data and a 10% growth on the log file.
I know that these settings should be adjusted to the specific customer, but I have not yet been able to find any rules of thumb about the optimal db settings.
In which way do you tweak the db settings ?
The SharePoint configuration database and the SharePoint content databases, should in a production environment always be configured with full recovery. You might otherwise lose the ability to roll back the databases beyond the point of an accidental data loss. The SSP databases can be on Simple Recovery, which I think they also are by default. They only hold stuff like settings and the metadata search index, i.e. they do not hold any content from users.
You cannot allow SharePoint to create your content databases. The default as you have noted is 1MB default size with a 10% growth. If a database autogrow occurs during use of your database, all web sites inside that site collection will stop while the database allocates more space. Also you will be all but guaranteed to get database fragmentation.
It is better to avoid unscheduled autogrowth of the database. To do this you will need to create the database manually and set it to the expected size of the database on creation. A document heavy site should be 50 - 100GB and SharePoint should be limited to not exceed that amount.
This number does depend on the hardware and amount of data expected to appear in the site. Smaller site collections therefore do not need such a large database.