What weather conditions can be disregarded in the planning stage, in the TAF?
None of it. You should consider all the weather when planning a flight.
However, when we look at the weather, we note the winds and the anticipated runway, and the expected visibility--can we fly the approach? I look at the ceiling...it's not controlling, but it may affect what we can do, and it's a factor in selecting an alternate, too. Any temporary (Tempo) should also be considered.
I'm interested in forecasts of convective activity, and the area weather, too.