I'm putting a vertical back up for transmitting and will
probably receive on my horizontal dipole.

Why? Doesn't make sense. If the vertical is best for
transmit, it should be the best for receive 98% of the time.
And visa versa.. When I ran both a dipole, and a GP,
I almost always transmitted on the antenna that I received
the best signal to whatever path I wanted to work.
If you run a good vertical for DX, and receive on the dipole
you will lose half the advantage of running the vertical
the way I see it.

The vertical
will be mounted on the top of a 30 foot Rohn 25G tower
with the base grounded in cement. The question arose in
my mind whether to connet the drooping elevated radials
to the tower or not. Does anyone else
worry about such things?

I don't think it matters too much as far as the operation
of the antenna. Either will work. But almost all my GP's
had the radials attached to the mounting plate, which in
turn was mounted to the mast. So all mine had the radials
connected to the top of the mast via the mounting.
Same as most any commercial ground plane...
MK