Witr and Nawaafil

Performing the witr before going to sleep

Q: It was the practice of Nabi (sallallahu alaihi wasallam) to read his witr salaah after he slept and woke up for tahajjud. I would like to follow this Sunnah, however there is fear that I may not wake up for tahajjud and miss my witr. 

Is it permissible and will I get the same reward as following a Sunnah of Nabi (sallallahu alaihi wasallam) if I delay my witr till before I go to bed, read my witr and then sleep? Do I get the reward of following Sunnah?

Occasionally omitting the sunnah ghair muakkadah before Asr and Esha Salaah

Q: We know it is sunnah to read 4 rakaat salaah before the Asr and Esha Salaah. Will it also be sunnah to purposely leave out these sunnah salaah SOMETIMES, as it is Sunnah Ghair Muakkada. 

Ezample, a person reads the 4 sunnah of Asr on Monday and purposely leaves it out on Tuesday and reads it again on Wednesday. Will this be regarded as a sunnah?