The "knowing what you're doing" part is the most important thing. Software alone won't make just anyone good at sign design or graphic design in general. An analogy: very few people are honestly good at singing; most people don't have the pipes and talent for it. The same thing goes for graphic design. But for some odd reason since computers are involved in the work process everybody and his brother thinks they can be an artist, despite the factor of talent. Sign design is a very specific niche. Even a lot of otherwise good graphic designers are totally out of their element when trying to design signs.