My opinion differs from theirs... During initial tank fill up, I think it's OK to use tap water. Some things to look out for... As Weberian stated, you don't wanna add phosphates, that alone would be worth using RO. Another is if your house is old and you have copper pipes... I wouldn't risk it then either.
Water dept. add a lot less chlorine in the winter months, so that's not that big of an issue.
I would premix the saltwater in 15 gallon tupperware storage bins in order to get an accurate salinity, so you don't hafta teeter totter back and forth. Or you can post the inner dimentions of your tank and we can tell you EXACTLY how many gallons it is, so you will know how much salt to add. If you do that, only give the verticle dimension from floor to where you want your water level to be, not the top of the tank.