Hoses on mains cold are carrying chlorinated potable cold water.
In all other cases the water is not exposed to the atmosphere or is cold
(storage cistern) where it does so.
The risks must be vanishly small, i.e. Other risks are far more
significant, e.g. using the stairs or driving a vehicle.

I suggest you read up about Legionella and its ability to grow in
biofilms. A Google search on legionella will show this Many unapproved
hoses have been tested and found to have rough internal EDPM surfaces
that have pores in them that hold biofilms. The chlorine in the water
cannot reach into these pores. Even the correct disinfection
proceedures cannot reach them.
Also have you never heard of dead legs in pipwork or underused pipe
runs? With respect, your supposition that going upstairs or driving a
car pauses a greater risk of Legionnares Disease would be laughable if
you were not a professional. Please treat this seriously in the
interests of your clients.
Also, as you are a registered gas fitter, may I presume you may also an
approved plumber? Were you aware that you could lose your approved
status by fitting non-approved fittings?