Selena, the photo above was sent in by a reader, Sarah, last January - we included it in a post which you can view here, where she talks a bit about the artwork in the collection. We think Sarah did a fantastic job with the clustered frame look.
Here are a few more posts (with photos) from our archive on the subject:
How To: Hang Artwork in Clusters
Clustered Artwork Revisted...plus two cool bedrooms
We think it's ok when these arrangements just grow organically, with no other rhyme or reason for placement other than what looks to good to the arranger's eye BUT were sure that Selena would love to hear some good tips from the readers, so please add your ideas and frame clustering wisdom to the comments below...