I bought & installed a tach on my 05 and was pleasantly surprised to find that the idle I already had the bike set at was right at about 1,000`-1,200 RPM. Out of curiosity, I turned it down to the thump, thump, thump, which sounded pretty cool, but I would NEVER keep it that low knowing the oil/head lubrication issues that it can cause.
In my opinion, if you think it is humming along pretty good, I think you've probably set it to a safe idle level already. I wouldn't bother with installing a tach, unless you like your gadgets...