Some thoughts on a workflow within
DeleD, the world editor I'm using to build my levels with for Discretion.
1. Build world
2. Triangulate world (divide non-triangular meshes into triangles)
3. Apply materials and fix U/V mapping where necessary
4. Export
It's important to do as much modelling as possible in 1. before proceeding to the next steps, because the easiest way to ensure everything is triangulated once and only once is to select all objects then do triangulate. It's necessary to do 2. before 3. because U/V mapping a non-triangular polygon doesn't always work too well.
The Wavefront .obj exporter for DeleD has an extremely useful 'scale' option which allows you to model at the scale DeleD was designed to be used at instead of at real world (game) scale (mine is 1 unit = 1 meter which in DeleD is way too zoomed in). The triangulator in the exporter is broken however, so make sure you triangulate using the geometry toolbox instead.
Hope this helps someone!