Yeah, you really shouldnt lay it on its side, as the liquid will feed into the reg, and if you have ever shot a paintball gun, you know that CO2 gets really cold when it sublimates. That is also why you would not want a CO2 tank with a dip tube, but most do not have them unless you request it I dont think (PB guys would request this).
Now, I do believe it would be fine if you layed it at an angle, where the liquid would not reach the reg, and had 'air space' to act as an expansion chamber and allow the liquid to become gas. The other option is an inline expansion chamber, but I'm sure that is not what you want to deal with.