The previous recommendation for emergency descent following depressurisation used to be to slow up to gear lowering speed, extend gear, then commence emergency descent. It gave a higher rate of descent though with a delayed start. Getting down to 10,000' there was not much time in it. This method is not recommended anymore as the current recommendation is to start the descent as soon as possible. Therefore, for Boeings, there is no maximum recommended altitude for gear, but note should be taken of maximum speeds in IAS and Mach.