If you're not using Asset Management and/or CMDB, I would suggest you delete retired computers, as there is no harm in doing so. If you mistakenly delete an active machine, it'll just recreate within the system the next time it sends inventory. If you have implemented Asset, I would NOT delete anything.
You can find Altiris' licensing model here (for NS6 only I think):
kb.altiris.com/article.asp