They should be fine.
I would suggest against using risers unless required, as they can cause reliability issues.
Source: Extensive experience building dense, multi-GPU rigs for password cracking, including fairly reliable "impossible" systems such as 4x 6990s in a board without extender cables.
Password cracking must be different than LTC mining, because there's no way in hell you could run 4x 6990 for LTC mining while crammed into a case with no space between the cards. Not unless your ambient was like 0F, and you had some delta fans blowing air between the GPUs. The only 4x GPU rigs I've seen that are still in a case are watercooled.
I've run 3 GPUs crammed next to each other inside a nice gaming case with lots of airflow. You pretty much HAVE to undervolt, and run the fans on max, or else the middle/top cards will overheat. Also, blower fans work best in this situation, as they dump heat outside the case, not into it (and by "best", I mean "slightly less shitty").
Working with risers isn't that hard, esp if you get those new USB ones. They are more reliable, and are pretty flexible, so you can position the GPUs themselves wherever and however you want.