I try to avoid sanding seams as much as possible but some seams I cant avoid it. When gluing halfs of anything together, I do my best to line up the parts as best as possible, clamp, hold, tape the parts together, and then apply the glue. This tends to reduce how much sanding and filling that will be needed.
In the case of gluing two plastic parts together, when the parts are lined up, I apply a thin styrene glue, like tamiya extra thin or something similar. As the glue begins to melt i slightly press the two halfs together. This will press a thin bead of the melted plastic out and tends to fill in almost all small gaps. Once the glue and plastic are dry, I go back and lightly scrape with an exacto blade and/or sand away the ridge that has formed.
this keeps me from having to aggressively sand and avoids flat spots.
When it comes to larger gaps, I first try to see if I can add a plastic shim to the gap. It is easier to fill larger gaps with shims that will fuse with the part and not shrink. If this has not eliminated the gap completely, then comes filler.
When I have to use filler, I try first to get the gap small enough with shims that a liquid type filler will do the job rather then a paste type. My preference is mr surfacer in 500 and 1000. I also have thinned paste puttys to a more liquid state.
I brush in the liquid filler, wait for it to dry, then swipe with a cotton bud slightly dampened with the appropriate thinner that will rewet and smooth out the liquid filler. This usually takes 2-3 applications of brushing in the filler and then swiping with the cotton bud due to shrinkage and air bubbles. this works very well on where the wing meet the fusalage.
This technique saves all the surface detail and only very light sanding if any is required with 800 or higher grit paper. I can then go in and scribe any lines that were filled in if needed.
I find most puddy type fillers either dry too fast to work well or dry too hard and you have to sand with aggressive grits like 300-400 grit paper which makes for more sanding and repairing the surrounding plastic not to mention all the lost detail.
If I do have to go to a putty, I will squeeze some out on a mixing pallet and then mix with the appropriate thinner to extend the dry time and soften it enough that it is more like a gel or toothpaste consistancy. I find this works into the gaps easier and i have to use far less. When it is time to sand, this thinner putty does not require aggressive sand paper. I find I can start with 600 or 800 grit so it does not damage the plastic as much.
I also will tape off the areas near the gap i am filling with putty if I know I will have to sand it once dry to try and preserve as much of the detail as I can.