[support] Can't clear cache with drush (file permissions)

I'm getting file permission errors while trying to clear caches using drush.

FYI: This is on my own workstation. OS = Mac OSX drush version 5.8 Running as user tim with : linus:~ tim$ id ...,70(_www),...

apache is user _www

files involved are user:group _www:_www permissions are 664 on files, 755 on the directory, which is sites/default/files/js

I note that sites/default/files/css is being handled without error and has permissions of 775

I want to automate this process for the command line as much as possible so would like a solution that doesn't entail clearing from the site itself

Would there be anything wrong with changing directory permissions on sites/default/files/js to 775? (My workstation is closed to the outside world)

TIA