R/O Water Storage Question

[spawn]

When I make up my water I use 5 gal pails. 1 is a full time reservoir that never moves. The other 3 I use for mixing and water changes. When I'm finished a change I wash out & dry the pails thoroughly.
It seems that if I store water in these pails though for the course of the week there is is rise in the TDS from 000 up to 2-5, so I don't use it, so the question is :
Is this minimal rise in TDS because I must have missed a spot & if so is this water still safe to use because the rise would be due to some residual salt mix? I don't feel like wasting any more water.