A few of my rocks are against the back wall. While it is not detrimental it does come with some set backs. If the rocks are not secured well and your cleaning the tank and knock any out of place it may one crack the tank or two scratch it at the least. Drilling or glueing rocks would be recomended or at least be very careful while working in the tank. The second major concern is detritus settling in placing that are inaccessible by water flow or siphon causing excess nutrients. Both problems can be cured easily and just require some attention. My tank is an SPS tank and have been lucky to be able to successfully deal with both issues. On smaller tanks these are easy to cure on bigger tanks of would require more work.