A starter should never add much alcohol to a beer. The point of a starter is to grow yeast, not produce beer. Reduced starter wort gravity and oxygen contact encourages a different metabolic pathway for the yeast which reduces alcohol production and increases yeast biomass growth.
As pointed out above, low gravities of 1.020 to 1.030 are not going to produce a good tasting end product and decanting that spent wort off the yeast cake should be an improvement to the overall taste of the beer the healthy yeast cake is pitched into.
A discrepancy to the recommendation above is that if the yeast are going into a much higher gravity wort, the starter gravity should be increased proportionally. I'd say that if the wort was more than about 40 points higher than the yeast starter wort, you might want to step the starter wort gravity up so there isn't as much difference in the gravities.