Raindrops, you might be interested in this article on critical degrees and anaretic degrees: http://www.cafeastrology.com/criticaldegrees.html
0 degrees is a critical degree in a cardinal sign, but not in a fixed or mutable sign. Presumably a planet in a critical degree has more power and becomes more important in a chart.
You might also consider dwads. A dwad is a division of a sign by 2.5 degree increments, such that each dwad is assigned a zodiacal sign. In the most common system the first dwad has the same sign as the sign it is in, but there are other systems.
So a planet at 0 degrees Aries is in the Aries dwad. A planet at 29 degrees Aries is in the Pisces dwad of Aries. In signs where the Pisces dwad occurs part way through, you simply continue with Aries and the rest of the signs in order.
What this means is that a planet located in 0 to 2 degrees 30 minutes of a sign should more strongly exemplify the sign, because it is getting a double dose of that energy.
Some astrologers swear by Sabian symbols, but I've not had much luck with them.