An ohmmeter is very necessary for safety's sake, knowing you won't exceed your battery's ratings. Particularly important if you ever put your mod in your pants pocket next to your ...uhh...ahem...ohh... you know.
But also if you build some coils that work better than any others on a particular topper, knowing how many ohms they are makes it easier for you to reproduce them again and again.
It would be like mixing up a fine batch of DIY eliquid, not knowing what percentages of each flavor you used, and now cannot reproduce the wonderful flavor in the next batch. Measurements are important.....
