Is it necessary to degass the resin before starting the infusion?

I've never done it ... at least IN2 degasses perfectly while infusing .   I have done both small and large parts, with close to zero pinholes without degassing ..

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Yeh, not necessary in my opinion. It complicates the procedure as well, so I would only suggest doing it once you are quite comfortable with the infusion process. 

if you have problems with bridging you willl have huge voids. The bridging needs to be addressed. If there are very tight radi and still possibly bridging despite best efforts then degassing benefits you in reducing the chance of voids in these areas too.