The Component/Parts placement in PCB for future serviceability
I don't think there is a standard. It is highly personal. I personally have found that 100 mil pitch on 0805 components is slightly too crowded for me when manually placing the components. Doable, but I prefer 110 or 120 mil pitch nowadays.

Also: why will you service you board? If properly designed no component should be expected to fail. I would suggest: don't overoptimize for space and gain some experience about what /needs/ service while you also gain experience in what you own service minimums are....