Should I enable Trim for SSD drives in Windows / VirtualBox? I understand that Trim exists for OSX Mountain Lion, but is there anything I should do for Windows on Virtualbox in this regard?
No. The Mac OS handles the physical drive - so any sort of second level defragmentation, optimizations or attempts of a guest OS to control the virtual file system driver would just add delay and complexity on top of the normal IO. Unless you were booting Windows directly in control of the SSD, this setting should be off.