dont remove any edges that are required to define parts of the model..

if you do this your model will have some oddly shaded areas, but if the edge you are removing co-planar to other existing edges around it, then its fine...

generally you would want to remove unecessary lines, but it doesnt really matter much in the end, it just slightly racks up how many polys you have in your scene..

three sided polygons arent bad, generally you want to be consistent with whether your scene is contained with triangles or quadrilaterals, but again, it doesnt really matter...

Triangles are only a big deal if you are modeling for a game, because all polygons must be co-planar, and all triangals are co-planar, so you wont get errors, but otherwise its fine...