IT0019 - delete a task type which has been used

Hi group,

One of our task types in Monitoring of tasks is obsolete.
It has been used before, so there are many records in IT0019 with the task.
Can I delete the task type, or should I leave it in and mark it "Obsolete".

(I realise there is a danger that we by mistake could reuse the ID-number if I delete it. )

Regards
Kirsten