To set the expectation with testers as to what makes a valuable bug, consider the scope of the cycle and what type of issues would have the greatest impact on quality.
These are less important bugs. They are typically low impact bugs that you may or may not fix, depending on priority and workload.
These are your should-fix bugs. Very valuable bugs are important, but perhaps not essential, that may be able to wait for a future build to fix.
These are your mission-critical, show-stopping and high impact bugs.