Myth #1 Being perma flashy is the same as being GCC all the time. Oddly, this was something that I thought for the longest time--that perma-flashies had to constantly deal with gate and station guns. However, gate guns will not shoot you if you're perma-flashy, only if you're GCC. The only thing that is similar between perma-flashiness and GCC is that people can shoot you without getting GCCed themselves.
Myth #2 Being perma flashy means you will get CONCORDed in high sec. I actually thought this until very recently, and had to have a lecture from a corpmate about all the game mechanics surrounding piracy. (Throw being in faction war into the mix and it's even more exciting.) You actually get shot by faction police in high-sec when you're perma-flashy. And yes, they will kill you, but not to the extent of being CONCORDed. In many cases you can buzz around in small ships and even kill mining vessels in belts before the police will show up. The only place you will get CONCORDed as a pirate is in CONCORD owned space.
Myth #3 Being perma-flashy greatly inhibits your ability to pvp by making it harder to reach common trade hubs. Well, isn't that a load of nonsense. If you really need to go to high-sec, it's very easy to use an alt, or have a friend grab something while they're there. Not to mention that there are many low-sec trade hubs that are stars above null-sec hubs.
In the time I have been perma-flashy, I have never had trouble getting stuff. In fact, my hanger is always chalk full of anything I need, and a lot of times things I don't really need. Yet, not being able to easily go to highsec is one of the biggest turnoffs of piracy for people. (Understandable if you want to live in high-sec or do high-sec wars, but more just laziness or empty excuses for other people.)
Myth #4 Being perma-flashy limits your pvp. Actually, it's the opposite. Being afraid of maintaining a certain number for your sec status is often what inhibits pvp--you end up not aggressing something you could easily kill, or not going on that small gang roam where you know priacy will be taking place.
Myth #5 Pirates are better pvpers. I have to throw this in the myth list because there have been many, many situations in which a neutral gang has fled when they could obviously steam roll us, due to the scariness of our flashy redness. The truth of the matter is, people can be perma-flashy for many reasons--they could have smartbombed in Jita for all you know. Just because they're red doesn't mean they're great at PVP.
Myth #6 All Pirates are douche bags. The other side to this myth is that the NRDS anti-pirates are the 'nice guys.' The truth of the matter is that no matter what you do in EVE, you'll find nice people and douche bags whether they're miners or pirates or null-sec FCs. Being flashy red doesn't mean someone is going to lie, cheat, and/or steal from you. Though, it may mean they'll kill you if they get the chance.
Myth #7 Being a pirate is practically permanent. Yeah, if you go all the way to -10, it can take a while to rat your sec status back up to respectable levels if you decide to go back to being non flashy. However, it's a lot more doable then people think. It's usually pretty easy to rat around in NPC null-sec, or even remporarily join a null-sec corporation to rat up your sec. It's actually harder maintaining a -10.0 sec status, as killing just one rat bumps it up quite a bit. (And it takes killing a lot of neutrals to get down that far.)