A few types of IP SLA's that are useful in a VoIP deployment:
UDP Jitter - This one goes without saying, as it probably the most common IP SLA that is deployed in a VoIP network. After all keeping track of the jitter within your network could be the first sign of a circuit/WAN/LAN issue or possibly a QoS policy that needs to be looked at.
DHCP - This one is not specific towards the VoIP infrastructure but hear me out. The VoIP phones probably won't be rebooting too often but if the endpoints are not able to receive their proper IP addresses in a timely fashion you will definitely will receive some tickets from the end users. Without an IP address those IP Phones are not really IP Phones are they.
DNS - Like DHCP, this one is not specific for your VoIP infrastructure, but if your IP phones are configured to perform DNS lookups to find their Call Manager or to utilize any specific services. Your VoIP infrastructure is more than likely dependent on DNS, keeping an eye on DNS performance could definitely give you a troubleshooting edge.
Historical IP SLA Information can save you!
Having historical information for IP SLA statistics can be just as useful as the IP SLA tool itself, after all having basic monitoring to prove network availability is one thing. Being able to provide provide performance statistics at the application level is another useful level entirely.
The historical information can also be used to identify and/or track peak usage times within the network, especially if you can see a performance degradation everyday at a specific time.
Yes, you can have too much of a good thing!
IP SLA's can be great for monitoring a network and provide a lot of valuable troubleshooting information, however you will definitely want to keep in mind the amount of SLA traffic you are generating (especially if you are marking all your UDP Jitter operations as EF/DSCP-46) this could generate issues in itself by wasting precious space in your priority queues if you designed your queues pretty lean.
Something else to consider, if you are terminating all your IP SLA's to a single router you may want to keep a close eye on the resource utilization of that router. The IP SLA process doesn't utilize a lot of resources but multiple that by 50 or above & it will definitely be a performance hit, & even possibly throw off the IP SLA statistics/results. In worse cases if the CPU/memory spikes often enough you could start seeing issues in your data plane forwarding.
Like everything else in life it is a matter of finding a good middle ground that provides you the monitoring you need without having any negative effects. So to what extent are you utilizing IP SLA in your network and what operational types are you relying on? Are you even a fan of IP SLA's? (I know I've definitely got some horror stories, but I do also have a lot of good stories)