According to @JCPedroza in my previous question about pop filters :
It (the pop filter) protects the microphone from things like fast moving air and saliva. Its main function is to attenuate the aspirated plosives of the singer, which can produce noise and strong ugly transients.
How about for guitars? Is a pop filter necessary? Do I have anything to lose or gain by defaulting to using pop filters whenever I record?
You should not use a pop filter when recording instruments, unless the instrument is air-powered and your mic is in the line of fire (and if that's the case, there may be better mic placement options).
The pop filter is meant to be as aurally transparent as possible, but it is indeed an obstruction that you are introducing between the instrument and the mic capsule. You don't want to have extra obstructions when you don't need them.
Pop filters should only be used when recording vocals. The purpose of a pop filter is to lower the "air-streams", that otherwise would be recorded by the microphone, which come out when you exhale air while singing.
However, if you record an instrument, there is no air that your microphone could record (at least not when playing guitar) and you are safe to use a mic without a pop filter. Most dynamic vocal microphones already have a built-in pop filter, which makes the microphone unsuitable for guitar recording (a Shure SM58 sounds very muddy if you use it for a guitar amp). Although, when recording acoustic guitar I recommend you to use a condenser microphone without a pop filter on the front.
Basically you gain nothing by using a pop-filter when recording guitars, but if your microphone already has a built-in pop filter, it's not a huge problem anyway... If you really want to record the best sound possible, you could save some money for an instrument mic (Shure SM57 or any "flat-top" mic sounds great on an amp!) or leave it as it is.
You could check out this article, if you really want to learn how to record a guitar amp correctly.
