Yes, mooee, the general consensus is that you should back combinations to win the same amount i.e. more on short odds and less on long shots. Incidently I have written an excel formula to calculate what quinella odds should be, quite handy.
However, I still contend that you should back 30 two-horse quins than 10 three-horse quins. By boxing three you are throwing 3 units in the pool and only hoping to collect on one unit. The thing is that it takes three times longer to rate 30 races than ten, so the time-poor punter likes to get as much action per rated race as they can.