Generally most engines triangulate every model/polygon before sending it to the graphics card, because tris are the fastest to calculate. So it not that important.
However I once had a bug because of a "stretched" Quad that wasn't triangulated in the right way. So it's a good idea to triangulate your model yourself when everythings done. That way you avoid unintended edges, and even save a tiny bit of rendering time because the engine doesn't have to do it.
Typically, it's best to go with quads. They're better for deformation. Though, you can use a combination of both quads and triangles without issue. Especially for BFME, where the poly count is rather low - to say the least.
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Also, most automated smoothing and reduction algorithms are made for, or based on quads. So quads should be mainly used for the modelling process. The occasional tri in a corner shouldn't be a problem thoug.
- Nazgûl likes this