DMM's work fine and are more accurate, generally speaking. Those little carto/atomizer readers though are handy. I use mine a lot more than my DMM, and like a previous poster suggested, they make a nice platform to build you coils on. So, no you don't need one. Yes, they are nice to have. I also have the 510 voltmeter, and find it very useful as well.
In case you or someone reading wants one of those carto/atomizer readers, they are hard to come by, but Exhale Vapors has them in stock now:
Carto Ohm Meter.jpg Ohm Reader - Battery Operated
Use coupon code "Delicious" for 16% OFF EVERYTHING! These go in and out of stock quickly, so this may not be available by the time you read this post.
votage meter 2.jpg The Voltage Meters are nice also and can measure "under load." Very useful.