I agree with Craigs first point. Except the Idil Time out means that the application will be unloaded from memory if there is no activity for 20 minutes. When someone accesses the site after this time then you get the site fired up again and the Sitefinity Int process has to go through it's start up.
For the Regular time interval I would not suggest to set it to zero. Allowing the app pool to recycle helps clear out any threads that have become orphaned or memory that has not been able to be cleared by the GC.
But I would recommend to set it to a particular time. A low activity period such as 1 am every day.
This won't cause a Sitefinity init process because IIS actually fires up a second process and directs all new requests to it and closes the old one when its finished. But during this process the application is not unloaded from memory.