javascript_include_tag :cached => "all" permission problems

I'm having problems with cached files. They are being written using
group "wheel" rather than group "www", which is what the other files
are all created with.
Also, since the permissions are 600 on these files, the web server
(phusion passenger) runs as my userid, group www (this is what I
desire, and what config.rb has as its owner/group)

Is there any way to specify a group to create files as, or a way to
change it very quickly, after creation? If that doesn't work, a way
to specify a umask for these operations would also work.

I'm not certain why my sever is creating files with group "wheel"
though. I swear this wasn't happening until recently, but this is the
first time I've spent effort to actually optimize file loading.