For a regular trader, exchanging altcoins to dollars or btc is just another tool of earning money.
Exchanging your coins to dollar in decreasing market would earn you money as well as buying back coins on an increasing market.
But you should be able to decide when to jump in and jump out. A planned and calculated action would yield better.
Yes that is a very good idea but when the market is going down then in that situation we do not have to think for ourselves but we have to support Bitcoin and our coins by holding them. If we all will hold them then the market will not go down and our assets will not face a low value.