"Dick's comment.... "There is a standard recommended practice on how to deal with empty pesticide bottles and any used spray equipment."
A few thoughts from a commercial pesticide applicator.
Never use the same sprayer for herbicides and other pesticides. The risk is too great.Label your dedicated herbicide sprayer in large letters  "herbicides only"   in case your son in law decides to help out in your garden.
The requirement to triple rinse containers is to reduce the concentration left on the container for safe disposal of the container.The container never becomes "clean". The rinsate should be added to spray tank and never dumped on the ground. Keep in mind the total water and concentrate ratios if you're mixing small amounts to keep the dilution rate consistant with the label.Further reading:
http://grounds-mag.com/mag/grounds_maintenance_treat_dispose_chemical/
The main reason for rinsing sprayers is to keep the residues from gumming up the works(after the water evaporates) when storing equipment. Wetable powders are notorious for plugging nozzlesand spring loaded ball valves and equipment shoul be cleaned immediately after use.In practice,however,sprayers in constant use are rarely rinsed UNLESS you are switching chemicals and there may be a compatability or plant sensitivity issue. Read the lablel.Of course ,common sense must be used. If you are switching from an insecticide to a fungicide you should rinse the the sprayer if there is any possibility of spraying plants in bloom.Any rinsate created must be treated as a pesticide mix and not just poured out.The approved method is to spray out on targets consistant with the label.This also rinses the valves,hose and nozzle.So ,with roundup,spraying your gravel driveway is  perfectly acceptable.
Some pesticide formulations should not be exposed to freezing temps.  Read the label!Use care in storing sprayers in freezing temps.Contact the manufacturer for guidelines.
Note:     These are just general guidelines. There are commercial cleaners available for specific pesticide formulations.Contact the manufacturer of the pesticide  for further information.

Jack



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