Most users won’t need to capture the IP of every signup. If you’ve had an issue with spammy signups in the past, or your business simply calls for an extra level of documentation, then the Signup IP add-on is right for you.
The regular signup timestamp, that's collected by default, is usually enough for most other users. But, it can't ever hurt to have an extra level of detail in your record-keeping, if you feel you need it, at all.
And here's more information about signup forms in general.