I don't think we should look at malefic aspects as an indicator of left-handedness, since left-handedness isn't a defect, as has been long assumed by many cultures. It's a difference, that's all. If we resort to using inaccurate cultural biases and forcing astrological conclusions to fit them, then astrology would then indeed be a psuedo-science, as its critcs so often claim.
I personally have failed in finding an easily deduced commonality among left-handers. Sun signs vary widely, as do most other factors, such as aspects to Mercs or Merc placement/dignification. I had the most success with Mercury in mutable signs, regardless of sign sign. However, even that isn't entirely consistent, just fairly common. Most notably I was able to find frequent occurences of left-handedness in Cancers with Gem Mercs and Aquas with Pisces Mercs, although not all Gem and Pisces Mercs I looked at were lefties. Lefties with Sag and Virgo Mercs seemed common too, but I also found several lefties with Cancer, Leo and Libra Mercs as well.
Then there was other variables that tend to complicate this matter. It appears less and less than peeople are simply right or left handed, but are on a broad spectrum that includes a number of possiblities. People who think themselves right handed, for example, may be more lefthanded than they assume, simply because they've never trieds to develop their lefthandedness as they have their right, and vice versa. Then's there's the obvious variations. I'm a Gem Sun with a highly dignified Gem Merc, but I'm not left-handed - I'm ambi. Granted, gemini is associated with ambidexterity, but it appears that most Gems are not ambi nor most ambis Gems. There's also a condition called "mixed handedness" which is often assumed to be ambidexterity, but it's a little different: in mixed-handedness, the brain assigns different tasks to either hand. So rather than being able to do most tasks with either hand, as with an ambi, a mixed-handed person may, for example, write with the right while using the left for opening doors or using keys. I had insufficent data on people with either apparent ambidexterity or mixed-handedness (mostly because people with either of this don't always know exactly what they have and resign to the role of a right-handed person), so I wasn't able to begin to hypothesis on this.