You should test more Y-DNA markers when you want to compare additional markers against others with similar results.
Additional STR markers will refine your matches. If you currently have many Y-12, Y-25, Y-37, or Y-67 matches, you should certainly consider upgrading your Y-DNA profile to a higher level.
The level that you choose and the need to upgrade depends on:
- Your goals,
- The amount of testing already completed by others in a Group Project that you are joining,
- The degree of certainty for a relationship (match) that you desire.
For genealogical matching, the most important factor is the degree of certainty that your near or exact matches are indeed related to you in recent generations.
Generally, testing additional STR markers will:
- Narrow the expected time to a common ancestor with an exact match,
- Increase the degree of certainty for a near or exact match,
- Reduce the number of irrelevant matches.