Every brewer has different thoughts on sanitation. Some people are OCD and sanitise everything in the room they brew in from the ceiling down, others are very lax and are quite happy to rinse equipment with water and accept that they get an infection once in a blue moon.
Personally we don't sanitise our caps when we bottle (we do sanitise our bottles though!) and currently have an infection rate of zero on our home brews, but we do understand others will want to sanitise the caps we make. To that end here is what we recommend:
We have tested iodophor, Star-San, alcohol, and bleach without any issues. These solutions should be used only for the short time necessary for them to work, and not overnight (long soaking can rust the edges of the caps). Use the proper amount of solution; high concentrations may have poor results (excess iodophor can stain, for example.) Use of other solutions is untested, and is done at your own risk. Some customers have had problems in the past with 'misting' of the gloss layer we put on the caps when using starsan, this will settle down itself and will
Boiling may soften or detach the PVC liners or gloss protective layer, so DO NOT BOIL your caps.
Some solutions may have a detrimental effect on the protective layer we put on top of the ink for your design. The best technique for sanitising is to use a spray bottle and place the caps with the printed side down. Then spray the internal liner of the cap that will come into contact with your brew.
Above all the main thing to do is test a method that works for you, one cap at a time. If you want to test soaking the caps in star san for 5 minutes, do it one cap at a time until you know it works without an issue.
Believe it or not, a lot of breweries do not sanitise their caps at all...