If you only occasionally run Bitcoin Core, then it is less urgent, though it would be best to upgrade as soon as convenient.
A bug was found which allows anyone capable of mining a sufficient-PoW block to crash Bitcoin Core nodes running versions 0.14.0 to 0.16.2. Stored funds are not at risk.
Since most altcoins are based on Bitcoin Core, I would guess that many of them are also affected, though I don't have any specific info on that at this time. Also, Core derivatives such as Knots are affected and have their own updates.
Main release thread: https://bitcointalksearch.org/topic/bitcoin-core-0163-released-5032424
https://bitcointalksearch.org/topic/dos-in-bitcoin-core-update-required-5032443