If you're talking about a Service Plan, meaning changing oil, etc, offered by the dealership...then that depends on whether you want the dealership doing all your service work, or if you prefer doing it yourself. I personally would not purchase one of those from my dealer.
What the dealer calls "Extended Warranty Plan" is actually an "Extended Service Plan"...it is not a factory warranty extension, but rather an insurance plan that takes effect after the 2 year manufacturers warranty expires. There are various versions, with the most common being a "5 year" (three additional years after the 2 year factory warranty), and a 7 year (5 additional years). You can get tire and wheel coverage for an additional cost with either/any of them. Whether it's something you ought to do depends...TECHNICALLY, if you do any engine mods or exhaust mods, it voids your engine warranty. BUT, application of that decision is somewhat dependent on your dealer...in other words, HD is inconsistant in their enforcement. The more modifications you do (cams, etc) the more ammo you give them to deny warranty coverage without a battle. Federal Law mandates that they prove the modification caused the failure, so you can follow that to it's logical conclusions.
Having said all that, I bought the 7 year ESP along with tire/wheel coverage. I do all my own work on my bike and just keep good records to prove it. The dealer only touches my bike when there is a warranty issue I feel is worthy of getting them involved in. I look at it this way: if I lost a wheel due to damage (not a wreck), or my radio took a crap, I've practically paid for the ESP right there. If you lose a lifter on the engine, it will likely pay for itself there as well, IF the dealer doesn't give you any chit for "modification" of the engine by virtue of an exhaust change and a tune. It's an insurance plan...you hope you'll never need it, but if you do, it might be worth it. It gives me peace of mind, so I bought it.