From the horse's mouth (Colt) -
From the Colt Safety and Instruction Manual for the MkIV/Series 80 & 90 Pistols:
CARRYING MODES
NOTE: This pistol may be carried in any one of the following three modes depending on your needs:
Mode 1 - MAGAZINE EMPTY, CHAMBER EMPTY
Pistol cannot be discharged. Use Mode 1 for storage, transporting, cleaning, repair, demonstrating and dry practice.
Mode 2 - MAGAZINE LOADED, CHAMBER EMPTY, HAMMER DOWN
Pistol cannot be fired until slide is cycled and trigger is squeezed. Use Mode 2 when carrying the pistol ready for use.
Mode 3 - MAGAZINE LOADED, CHAMBER LOADED, HAMMER COCKED, SAFETY ON
Pistol can be fired when slide lock safety is off and trigger is squeezed. Use Mode 3 when you must be prepared to use the pistol immediately without warning.
And in another section:
CAUTION: DO NOT CARRY YOUR PISTOL WITH THE HAMMER DOWN ON A LIVE CARTRIDGE. To do so means you must lower the hammer. To lower the hammer, you must squeeze the trigger. When you squeeze the trigger you disengage the firing pin lock. This is not a safe condition. Instead, we urge you to follow the instructions in this manual. When you do so, you can enjoy the safe use of your pistol.
Note that Colt's Mode 1 is what is commonly referred to today as Condition 4, Mode 2 is Condition 3 and Mode 3 is Condition 1.
This is, of course, a 1911 and has no decocker. It is an example of a recommendation against operating the hammer with a round chambered.
The Sig 220 owner's manual doesn't describe what happens when you manually cock the hammer. If their patented safeties are disengaged when the decocking lever returns to its "at rest" position, then manually cocking the hammer with a round chambered could be much the same as doing this with a 1911 with no firing pin safety. On the other hand, if the Sig has a firing pin interlock that the trigger releases, it's probably OK.
In general, I'm a little wary of doing anything not called out in the mfgr's manual...