If you're carrying collision/comp with a $1000 deductible and you have your amps/subs in the trunk and your speakers well buried in the rear deck, dash and/or doors, how much risk are you assuming compared to the price of the extra coverage? HU's are not that expensive unless you get one of those video screen ones and even those are mostly under $1K. If someone steals the whole car, they'll have the time it takes to dig out all your audio equipment along with all the other goodies but your garden-variety smash-n-grab isn't going to score the average thief very much. Used HU's don't bring in big bucks if they are just playing CD's since there's so many of them already being used, so thieves aren't going to give HU's a high priority if it's the usual "same ol'" HU.
Now if a person had a truck, a van, a hatchback or a 'Vette, then all the goodies are up for grabs and that being the case, if the system is worth more than the deductible and the inevitable hassle of dealing with a rate raise leading one to change companies following a claim, then insure the heck out of it plus put in an alarm system.
Calculate the cost of the policy rider vs the cost of what's at risk. Chances are good that you could set aside the money you'd pay the insurance company forever for a few months and be able to cover the swiped HU if that's all they can snag easily and who's to say digging HU's out of dashes is easy?
Rick