It's important to realize that not all models need to be welded together as one manifold piece (unless you're going to 3d print, or animate in a particular way). Typically, you would want to model an object the same way it might be built from parts, or fabricated in the real world, assuming the object is a real world replica. However, in all practicality, that may not be possible, or may not make sense for the situation you are intending to use the model. The best way to model is to always think of the most efficient way to achieve the look you want, while conserving polycount, and keeping clean topology.

Just a quick tip about topology too: A few triangles here and there may be unavoidable, and shouldn't pose a problem if they're limited to flat surfaces. Ngons, on the other hand, can be problematic in a host of situations, and should be avoided in the final topology. They are useful sometimes during the modeling process, but if your model is intended for redistribution, then ngons should be fixed.

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