What is the optimal response time for file growth
First of all you want to make sure that your database is sized appropriately up front.  Usually I like to plan for 6 months to a years worth of growth.  That being said your data file(s) should be the only ones growing on a regular/irregular basis.  Your log file once it hits a certain point should not grow much if at all, assuming you are taking regular backups.  Given that, you can use a server setting called Instant file initialization that will make your database growth very very quick, regardless of how much space you are adding.

Here is a link you can read to start you on it.  http://msdn.microsoft.com/en-us/library/ms175935(v=sql.105).aspx

If you have this turned on then you can afford to set your auto growth to larger sizes so that you don't have to grow as often.