Another daft question, but have to ask, while my 1995 Ducati 750ss is in storage over the winter months should I run the engine once a month or just leave it, I wondered whether running the engine might cause condensation within the cylinders?
I don’t usually run mine while laid up. It won’t cause condensation in the cylinders but it will build up in the crankcases if not run for long enough.
Use an Optimate but only overnight about once a week, that’ll keep the battery in good nick.
Kevin
Like Kev I use an optimate. I do run mine up to temp. Probably don’t need to but I like to hear them! 
I bounce the suspension and operate the brakes and clutch as well due to past issues with bits sticking, drying out seals etc.
I concur with both Kevin and Dutchy916.
Therefore to run the engine or not is a decision to make on what’s best and convenient for your circumstances.
If you do run the engine, make sure you get it thoroughly hot to drive off all moisture in the engine and exhaust system. Running the engine also has the advantage of keeping fuel circulating through the filters and carburettors to stop these gumming up.
I notice from an earlier post that you will be storing the bike in a perm bag. This may make it inconvenient to move the bike around and run the engine during the winter months.
If you do lay the bike up, it’s a good idea to have clean oil in the engine. Also, run the engine, with the fuel taps off until the engine stalls, to clear the carburettor jets. Then, drain the float chambers to remove any residual fuel. Also draining the fuel tank may be advantageous.
Personally, I like to ride during the winter but only on nice days.
I hope this is helpful.
Macer
…and move the bike to avoid flat spots developing on tyres.
It’s much easier to ride the bike than store it. 
Where are the fuel taps on a 1995 Ducati 750 ss
There should be a petcock on the underside of the tank. To the rear right hand side.