Not always no.
The depth of field at the distances you are at will easily encompass the aircraft. Obviously lenses are a bit better when stopped down 1 or 2 stops, but in poo weather that's not always practical. I wouldn't agree with using exp comp either, I don't, but then I only ever go to the Mounds, not the park and view, unless its very cold as they have a supply of coffee.
Personally, I'd go onto the mounds on the South (ish) side of the airport, when they are taking off towards the Irish Sea. The nearest to the threshold is the better place for yukky weather. otherwise about half way and take some take off shots.