It's a good idea on any plane, but especially on aerobatic types with huge control surfaces. If you are flying a .40 size trainer with a plain bearing (not ball bearing) .40 engine then you are fine without sealing. Any more power then I would seal. I purchased a huge roll of clear Ultracote to do side windows on an Aeronca Champ model. That roll of clear is only half gone after 10 plus years of gap sealing and other repairs.