I've seen several threads in the past couple weeks asking the same question.
Bottom line is that your tap water is most likely different than mine and different than many others. So what works for them may not work for you.
Also, the idea that "I've kept if for XX weeks and had no problems", may just mean that you have been lucky. Without testing it, you don't really know what has happened to the effectiveness.
Personally, I buy 2.5 gallon jugs of distilled water and mix StarSan right into the jug. I use that ONLY to fill spray bottles, nothing goes back into the jug that could carry contaminants. I spray most things A LOT to sanitize. 2.5 gallons lasts a very long time like this. I check the PH occasionally and have never saw it over 3. Typically a quart spray bottle of mixed starsan will last me for a brew day. So a quart of concentrated StarSan will last me a VERY long time.
Occasionally, I host a "brew" and have 5 or 6 guest brewers over. On those days, I mix 5 gallons of starsan with tap water for everyone to soak/dip/rinse their stuff in as needed. That gets dumped at the end of the day.
Bottom line, if you are gonna keep it, used Distilled Water to mix it.
It's cheap insurance... you're gonna kick yourself if you end up with a problem and after chasing all the possible reasons find it's because you tried to save a couple pennys.
Ed