Here is a list of running jobs on the slave I have checked /var/tmp on. The imageio job cleans /var/tmp and jenkins job doesn't.
Anyhow, I'll take your word on that and assume that the Jenkins build history has bugs and a VDSM or some other job run on that slave.
Now lets go back to the main interest of this thread. If we'll know, that whatever is being written to /var/tmp, can be considered as cache and can be used by the next run of the job that uses it, it might be a good idea not to clean /var/tmp. Jenkins is helping us with that by trying to run jobs on the same slave as much as possible.
We will start by monitoring our disks constantly to see how fast, if at all, they are getting full.