Yap, and not only when the prices of altcoins are higher, but you can also sell your altcoios if you have other better reinvestment plans, like investing in a new start up project that could bring in better returns (profit); and you have carried out proper research on it.
I think that there could many reasons to sell your altcoins other than profit margin. You can as well sell to reinvest also in peoject that has gain much wider attentions than the coins you already own. And finally, you could sell your altcoins if you have meet your goals in the business.