VMware wrote in their "What's new in VMware vSphere 5.1" document that starting from version 5.1 no reboots (Zero-downtime) would be required when updating them to a newer version on a Microsoft OS (Vista and up).
Soon after they William Lam wrote a clarification blog post on the VMware blog which explained in more detail under what circumstances Zero downtime was needed when updating VMtools.
The blog post informs us that there are less VMtools update actions that need a reboot, but there are still some drivers or other components that will require a reboot when being updated or replaced.
From the vSphere (web) client you can update VMtools automatically and with advanced options you can suppress the reboot (even if it is required to complete the update).
But if you want to quickly update VMtools on multiple VMs at once then the easiest way is to do it by PowerCLI, by using a simple one-liner you can update them and suppress a reboot.
If you want more details on which update situations require a reboot, please take a look at the VMware KB2015163