sendmail can kind of be a coin flip.
On the one hand, it's used as a local logging facility and, per its default configuration, will not even relay mail for any other machines. But, it's still used for logging and such. For that reason it's generally not "best practice" to disable it.
On the other hand, I've had it disabled on my netbook for months and the sun still rises and sets.
If in doubt, leave it on. It's not a huge resource-eater anyway.