Oracle installer enforces NTP up and running on all nodes where Grid Infrastructure is to be installed.
Slewing is an option for the ntp daemon (ie. ntpd -x to prevent time from moving backwards in large amounts. This slewing will help reduce time changes into multiple small changes, such that they will not impact Oracle Clusterware. While it has been a requirement for a long time with Oracle Clusterware, it is more strongly enforced with 11gR2 as the clusterware is more sensitive now to time changes. (This fact is also shown in the need for Oracle to create CTS (Cluster Time System) which can be created and used on the nodes of a cluster to keep them in sync with each other if NTP is not available or accessible).