Selling your bitcoins is not smart because of the fact that it can bring you in big trouble, I agree about the fact it can be good sometimes but most likely if you are selling to fast you can lose money or make less more profit than you could ever do in the future.
Selling bitcoins is good but selling all the coins you have is really not a good idea as the price might grow later on but holding all the coins is also not the best idea because if you dont sell any of it in a good price then it may drop again from that point and you will lose the chance of getting more profits out of it.
Yeah and only holding without selling them at any point doesn't make any sense as it will take opportunity away from you of making profits and if you are investing your money then you have to enjoy the profits too.