Chris,
Any files, templates, etc that are non AC which are vital to running the office, need to be backed up... and strongly recommend more than one location.
WD external is fine, but let's say, its damaged, stolen or lost.. what is the back up to the back up? An online back up system is a nice "insurance." Others have posted online back up options, for various cost options. I still use the AC back up, it is the lifeline to the practice... we can no longer funtion without AC working well. AC's back up AND support(magnificent Tech Support Team) are on my "must have" list.
You may choose not to back up your imported items EACH time you back up AC, but do so on some level, and make sure you always back up your imports somewhere... external hard drive( or two) and an online back up is great, its like a virtual safety deposit box at the bank... the advantage is it will be as updated as you choose.
BY the way, you said easy to use... AC... admin... offsite back up... make sure you have the right location (s) you would like to have local copies stored ( like your WD, for example) and press back up... I don't know of anything simpler.