Well I just implemented a new module in the system I develop.. I use replication to two slaves (MySQL) and the slaves are V5 the master V4, and once apon a time I thought the use of backticks was overkill.. untill today. Turns out a simple addition of a reserved word in V5 did not effect offcourse the master V4 but caused the halt of both replicaton servers that keep the running backups.. I eat my own hat. Use the backticks!
We had a situation once where backticks were required using Postgres. The reason, simply mixing upper and lowercase in table names. Hence we now always use all lowercase. Haven't run into any problems. I have heard ORACLE requires all uppercase characters for tablenames though...