It really depends on the status of the game and how you think it will go from then on.
For example if the status quo remains and the player can stack more in the capital then you can bring to attack, your odds are just going to go down, so even though you may only have a sub 50% chance to take that you may determine that is the best shot you’ll get. But if you determine you can out stack them and increase your odds then you wait it out.
If you are talking about a shot on UK or Ger (via sea), that’s a different story b/c those can be one-time threats and could have long term effects since they may interrupt your current transport system, so while it won’t lose you the war it can mess you up a few turns down the road. Now, I’d consider taking a really low % shot (30-40%) at either one if I felt that I didn’t get off to a particularly good start.
But I also think I have a good feel for the game and can judge whether I’m winning or losing. So, as the Allies I’ll pass up low % to hit Belin and just continue to stack until I have overwhelming numbers.
But as was pointed out if there are no consequences you might as well take a shot with a 1% chance. But that would seem to imply you have an overwhelming lead already.