i guess you are right the spreader matt would build up salts after some time, i havent had any problems with taste though .. perhaps it's the food you are using or another issue ?
i have always found the small tanks 2ft long or so dont really need the spreader matt, but as they get longer 3ft onwards the stream is centered and doesnt fill out to the edges, i use just a small square of spreder matt under each rockwool block to give the roots something to bite into
im sure you will be fine as long as you keep your eye on things, watch out for puddling, when the roots have spreader matt they do tend to grow in a matt and more evenly spread, if you get clumps of roots in 1 area and not another this can cause puddling, i would recomend that you run your pumps on timers anyway, so the roots get more chance to breath i have had more success with nft when running the pumps on timers, in the early strages when plants have few roots i find running the pumps all the time can cause problems for young plants, this is more of a problem for plants in rockwool that sit directly on the tables as rockwool gets very wet
in the early stages i would run the pumps on for 60 mins off for 60mins , and during flower on for 20 mins off for 40 mins in every 24 hur period
i dunno what type of pumps you are using if it is the standard nft type of 350-450 lph small pumps , keep a very close eye on them too, they do stop pumping from time to time, due to all the salt build up, i put 2 pumps in each tank and pump them out of the same outlet with a t-joint adaptor .. good luck