Yes, selling Bitcoin at a reasonable price is not a foolish decision. The stupid decision you can make with Bitcoin is that you can sell your Bitcoin for a low price or during a Bitcoin dump. Your sales can be one of the strategies to get fresh money to start a new trade so it really depends on how the user stands.
I don't think selling bitcoin generally, is something that could be labeled as stupid. What's stupid however, is selling bitcoin when the price is at a decrease rather than buying more or holding it until the price becomes a litlle bit more desirable for cashing-out. After all, selling bitcoin promotes a good cycle of pump and dump in the system. Which is a healthy thing to say the least.