If you are referring to buying the monthly service card--then I don't believe there is a penalty. However, during that time you may lose your phone number. But, once reconnected, you can get a new number.
Yes, there is never a penalty as you are not under contract and are paying for service in advance (or pre-paying for service).
However, the big disadvantage is if you let service lapse, you "may" lose your phone number. The word "may" is the language Straight Talk uses in their T&C and what their reps continually remind you over the phone but from what I hear through the grapevine is that they usually will give you 30-days before you actually do forfeit the number.
If having the same number isn't important to you, there is no penalty for skipping months.
Btw, I use a Google Voice number with my Straight Talk phone so that's an option you might want to consider. I can use my Google Voice number to call out and ring my phone on incoming calls in addition to my Straight Talk number. If you use a GV number then it really doesn't matter if you lose your Straight Talk number.
After a month you will lose your phone number.