do I need a locking gas cap

If you think for a minute that no one will knock a hole in your tank you are nuts.....I have seen it several times. Just like cutting off the catalitic converters.....Saw them cut off of 200 cars in one parking lot one night. Siphoning went out in the 60s----they now use a portable battery powered pump and can get into what you would think is safe tank pretty easy. Don't ever under estimate the abilities of a thief. Even at $2.00 a gallon 20 gal is $40......