I haven't worked with the centaurs, so I can't say. The rule is, understand what's going on with the majors before moving on to the minors. In synastry, I think the main indicators of a good romantic relationship are:
1. Suns sextiled or trined.
2. Moons conjunct, sextiled, or trined
3. "Reverse" Mars-Venus conjunctions, sextiles, or trines.
4. One of the above conjunct, sextiled, or trined the other person's ascendant.
5. Inter-aspects with the above. (For example, A's moon sextiles B's sun.)
6. Jupiter is a "feel good" aspect, Mercury is good for communication.
The above is a best-case scenario. Seldom does it manifest in its entirety in reality.
Squares and oppositions may initially give a lot of excitement, but eventually these manifest as stressful aspects. Juno is supposedly the "marriage asteroid," so that might be one to look for.
The modern outer planets operate at a generational level. For example, Pluto may move only a couple of degrees per year. So everyone born in a given year will have Pluto in the same place. Nessus has an orbital period of 122.4 years.
So a Nessus-Uranus contact would be way down on my list of things to look for in a chart.
Where I do think asteroids get interesting in synastry is when you find a "namesake" asteroid, and you can see how it falls in the other person's chart. Not everybody has an asteroid with their name, but when you do, they're worth exploring. For example, asteroid Hillary conjuncts Bill Clinton's moon. My husband's namesake asteroid conjuncts my sun. You can input any asteroid into a chart at
www.astro.com . You can use cognate names in other languages. For example, "John" might show up as Juan, Ian, Jean, Ivan, Johannes, Hans, Jan, &c.
I would also look at a couple's midpoint composite chart.