There is nothing you need to keep from a transaction that old. 

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.

The form 5498 should always be kept in your IRA paperwork file and should remain there as long as the account is open ... the actual tax returns can be destroyed after 5 years ... keeping a lifetime of tax returns is not necessary.