I would keep the 1998 Form 5498 from the Roth IRA forever. This form substantiates your basis in Roth conversions if you make a distribution from your Roth IRAs prior to age 59½. After age 59½ this form substantiates that you had a Roth IRA long enough ago that any distribution from your Roth IRAs would be a qualified distribution.
I agree. My general advice on how long to keep ANY tax related document is to let your heirs dispose of them.