There is no tone suckage if you're using a high quality unit with top shelf A/D D/A converters, e.g. like some units from TC Electronic , Eventide, etc.....
Some people confuse suckage with tone coloration....but coloration is a natural byproduct of effecting the tone (duh?). I found that when I had my Eventide Eclipse (which is known to have great audio converters...) the quality of my tone actually improved in some of my amps that didn't have the greatest loops (like the older Shiva).
Pedals from Boss will surely degrade the quality of your tone in the Mark V loop.....