I will mix. I didn't used to but those choose keep on formula longer, sometimes that child want it and sometimes not. If it come mixed, then times it get thrown away end of day.

Some parent mix. Some parent pre-measure water and send little divided container with pre-measured formula need add to water, some just send a can of formula. I not often take more than one little so not big deal, to me. Not take any more time than wiping bottom of potty training child. There time in day I spend couple minutes doing task for one particular child, I not see this as any different.