What is the best practice for setting container parameters that need to be properly setup, e.g. I have a parameter called 'feed\_filesystem\_cache\_path' (string of '%kernel.cache\_dir%' + /directory location) which I am injecting into some service classes. 

I see myself checking on existance in those classes and would like to know if there's a better way to do that.

I've tried to alter my bundle's container extension, but seems like the parameters aren't resolved by then; I'm still getting '%kernel.cache\_dir%/feeds' instead of a fully qualified/compiled path.

Trying in Bundle class, in the build method (just after parent::build($container) isn't helping me much either, at that point the containe only has "kernel" services and parameters.