It depends on your trading strategy. If you think the altcoins have potential and plan to hold for the long term, there is no point to sell if the price dip. You never know when the altcoins are rising again.
I would agree, It's up to your limits if you hold or sell you altcoins. As i can see currently there are altcoins that are doing well and have more potentials for a higher returns. If your coins are these coins then it is the right time to sell but it all depends if you really need the money if not then you may hold it.