"Нажмите кнопку, чтобы открыть дверь" uses the perfective, since the sentence looks like "do the SINGLE COMPLETE ACTION in order to other SINGLE COMPLETE ACTION happened". If you use the imperfective нажимайте, a reader will expect a continuation with an explicitly iterative context such as "каждый раз когда вы..."

"как сделать салат". Both perfective and imperfective are OK in a title, but the perfective is used far more often. The reason is: when a person is looking for that information, s/he is usually interested in having a dish made RIGHT NOW, not every day or such.


"Когда не хочется выбирать между сабом и шаурмой"
As Eric said, выбрать here sounds like "when you specifically don't want this selection to be ever made".
The imperfective moves the focus point from a result of an action/process to to the process itself.