If your SERIOUS about your install, YES.
{climbs up on soap box, and talk to EVERYONE}
If drilling a few holes is out of the question, then don't put a sub box in your car/truck/SUV. Besides the OBVIOUS reasons of theft prevention (which doesn't matter, if they can't get the subs for themselves, they'll make sure YOU can't use them either)...there is the (ever so slight) sound improvement you'll get from not allowing the box to counteract the subs motion. Theres the body or interior panel damage issue from having a box rolling around when you take a turn, accelerate, or brake....and LAST BUT NOT LEAST...There's also the SAFETY issue....in an accident, the sub box becomes a WILD, HEAVY MISSLE in the back of your ride. And depending on where the box is located, and how you are hit, that box could go ANYWHERE...
{climbs down off the soap box now}
My box, with subs, wiring and caps weighs ~250 lbs. My amp rack and wiring weighs another ~100 It could get DANGEROUS in an accident if the proper precautions weren't taken.
Ryan