When you have the gearbox out if the shaft is loose that goes into the flywheel that's when you know the bearing needs replacing and if your doing that one while the box is apartyou might as well do the rest! Your as well having a new clutch release bearing there as if it is that then you can get it swapped straight away. If you leave it, it could just cause the gearbox to sease up and damage more parts, so best getting it looked at