Inlet.

because it is pointless at outlet.

at inlet i can see 2 possible reasons.

1.to know pressure received at inlet
2. to know how dirty the filter is. if dirty, it will a reading that is a bit higher than usual when there is full water usage at outlet.

yes. but you should not wait for that la. i backwash mine every week. but i have very high water usage. yours may vary depending on usage and water quality 

when i said high means a few psi higher than normal when using water.

example :
when not using water it is 28 psi.
when using water drop to zero or almost zero.

but if you notice pressure is 3 or 4 psi when using water.......means there is restriction of water flow due to dirty filter.

Depends. Existing pipe water resistance.
Type of valve at kitchen or other areas which use water, normal water pressure value etc etc.

Not that it matters.

You just have to know the normal values of your system during water usage when the filter is clean. Any value higher than that would mean a dirty filter

Yes.

Value pressure reading at inlet during water usage with dirty filter will be higher than the reading at inlet during water usage with a clean filter