There are plenty of people who set up heavily planted tanks that do water changes only 3 or 4 times a year sometimes less. Topping off wont cause all that much of a change so long as you don't wait for half the tank to evaporate. This is even further reduced if you already have soft water, it will take a long time for the minerals to accumulate especially with the plants consuming some of them. If you want to be absolutely sure that nothing will accumulate, top off with RO/DI water.
http://www.plantedtank.net/forums/low-tech-forum/80371-super-low-tech-20h-new-fish.html
http://www.plantedtank.net/forums/low-tech-forum/80371-super-low-tech-20h-new-fish.html