I've tested this on both Homelite and Ryobi engines running Master Airscrew props (16x8 and 18x8 respectively).

On my Homelite (25cc and/or 30cc) I got about 600rpm more. On my Ryobi I have yet to measure the difference but, a friends Ryobi went from a vibrating mess to something far more tolerable. Note the vibration was most noticable at low revs and, the top end had about 500+ more rpm.

I'm no expert but for a single cylinder engine they are inherently unbalanced (you can only get so much balance in the design). So, by indexing the prop the essentially line up with the crank throw you are probably making only a negligible alteration to the overall balance. By not indexing you are potentially adding another plane of imbalance to the engine, which may either cause or magnify any existing imbalance... Well, that's my theory.

In other words, best to have the imbalance matching the rest of the engine. If you're using electric start, there is no harm in indexing the prop so why not do it.