I took my environment from 5.1.7 to 5.2.6. Followed the directions that were provided and it went without a problem. I then had the machines update by group across three weeks. Those on the old client still communicated keys/lockouts just fine.
you should review the release notes to determine if your environment needs any of the upgraded items for your clients. If not, then you don't really need to do a client upgrade. We found that almost always, there is a reason to do the upgrade.