believe it or not, that is a really great question, but one i unfortunately do not have answer to right now. but i am going to ask a few people who would. as i understand it, water softeners remove the alkalids from the water, thereby softening it. i guess the main consideration is what is used to soften the water. what chemicals are used in the process and whether or not chemical dechlorinators remove them. one could easily assume that if it is safe for human consumption, then it should be safe, but, just because we can consume it, doesn't mean it is healthy for a frog to absorb directly into their body. we at least have digestive fluids, kick butt antibodies and have built immunities to such things, frogs don't have those luxuries.

if you are doing small water changes, of say a couple gallons, there are some nice natural ways to soften your water. peat, is an excellent natural buffer. it's great for lowering pH and reducing alkalinity. there are peat filter packs you can purchase for aquarium filters for just this very purpose. if you were to soak on in a shallow pan of water (i'll explain why shallow in a sec), for a week or so, you should see a decent change. Carbon dioxide (co2) also lowers pH. since introducing co2 into water just for water changes really isn't economical, the more surface area, the more co2 you can add to the water. stirring it occasionally would help, but you're not really going to add much. this would be a negligible difference, if any at all. but hey, every little bit helps sometimes.. lol

i will definitely ask a few of my planted aquarium friends who are like the Rainmen of water chemisry and get an answer to your question though.