Trace Flag 1117

Should always be enabled, if you’re using more than one tempdb data file (which you usually should do :-)).

Trace Flag 1118

Should be enabled in OLTP environments to avoid contention (SGAM) by disabling mixed extent allocations in ALL databases. If enabled, the RDBMS will always allocate a new 64KB extent instead of trying to find a mixed one, in which smaller data might be stuffed. See also Book: SQL Server 2012 Internals and Troubleshooting pg 228.

Should also be enabled when using Snapshot isolation, because Snapshots are stored in tempdb. See also pg 332 and http://support.microsoft.com/kb/2154845

Downside: You may use more space (ok, not really, as long as you don’t have millions of users creating 10KB chunks…).