I personally do it once a year, before I change the oil (which I just finished doing). I do around 8K miles/year.

 

My Jag mechanic said that in "most cases" that he sees, it's due to owners getting low quality gasoline at non-big-name stations, ending up with low grade fuel.

 

My rough idle and smell was accompanied by white smoke. 2 blown injectors on one side. Replaced 3 on that side and all new plugs. Indie bought all Jag parts so I really only saved on labor, the only way he would give long guarentee.