I agree with Eric on both points - you don't need to use them but keep the stays away from the slot in any case.

As a counterpoint, I received a gift of a few boxes of various tubing. The builder had the tubes/fittings in his attic for 15-20 years from a previous production scale endeavor for brands and people we'd all likely recognize. Most of it was prototype bent chainstays and mitered tubes for abandoned prototype projects. There were a couple sets of seatstays that the tops were plugged and then machine mitered for fastback attachment. All of this to say that there is evidence of large-ish scale producers that use or have tried to use plugs in this joint. The large surface area provided by a plug of some kind, would be ideal for a thin layer of silver and no fillets as that is mainly a joint dealing with shear/compressive stresses.