How do I close this up? (Max'09 - Subd)

[Tiros]

Aslong as your highpoly mesh looks fine, there's no way there'll be any problems in your normalmap aslong as your lowpoly and cage is fine aswell

As said, triangles are relly not an issue on planar and non-deforming surfaces. In fact, triangles can be used on curves surfaces aswell, to chieve hard edges etc. in certain areas.. ofc, it's better not to use triangles if one can though.