TH looks like things are a bit different than when I first looked into registering service marks on line a few years ago.
Last go around there was no option to register the service mark, it was all labeled for trademarks.
So I stand corrected.
Still copyright protects the logo because any original work, no matter how it is recorded, is protected. that includes all art, written word, spoken word, music, video or any combination there of.
Since a service mark is such a recording it is covered by copyright law and given a good deal of protection.
Really all registering it with the USP&TMO provides beyond that is a defacto establishment of first use claim. With Government registration it has been settled who had it first. That can come in handy in some circumstances and may well be worth the fee.
As mentioned earlier though you have to vigorously defend any infringements. If you fail to defend it in one circumstance then you can lose the right to defend it in the future.
Also, every time you change the mark (I'm not sure how much of a change triggers the need to) you need to apply for new registration.
For example if my logo were a log and branch for Wood & Son and I add a flower for Wood & Son & Daughter I'd probably need to register it as a new SM. If we later put faces on them we'd have to do another registration. Not a huge expense but is it really necessary?
Since a service mark by definition is only used in advertising and documentation I've opted to just defend my copyright. You can add the (c) symbol to your logo which helps allow you to defend any use of logo by itself even if the logo is part of a broader document, although again, by def, you are entitled to copyright for any part of the work whether whole or excerpted.
From my understanding, failure to defend your copyright in one circumstance does not preclude you from defending in another as you would need to do for patent or trademark defense.
All this said you can use the SM symbol with your logo anytime you wish. You can also enclose a copy of your logo in a sealed envelope and mail it to your CPA or lawyer instructing them that you are establishing date of creation with them.
Should you ever have to defend it that is the sort of evidence you would want.
41 cents versus several hundred dollars seems a better way to go for me.
Besides, registering and copyright really only keep honest people honest. If someone uses your logo they have already proven they don't care about the rules!