Hmm ... it's always an interesting question, the bench! We all try and get as much as we can out of our starters, as they are our stars - but lack of bench depth can kill you in those close ones, as you've already found out. My own bench strategy is still a work in progress, but here's how it has developed:
Step 1: Work towards a 5/3/3 lineup. That's your five starters, 3 decent bench players, and 3 scrubs. Back when I was running for a D3 title in Australia, that's kind of what I was aiming for. The decent bench and the scrubs should both be broken down further as: 1 small (PG/SG), 1 big (PF/C), and one that can play SF (as well as preferably at least one or two other positions). Decent bench players for me then were around the $7k-$10k salary mark, scrubs anything under $5k.
Step 2: Incorporate a 6th man. Ideally, he is versatile and can play multiple positions - and ideally one of those is SF, so even if you suffer an injury or foul out your team can adapt and players can move up/down accordingly with the five solid starter players you have remaining.
Step 3: Gradually improve each player, if you find someone at a cheap price that is better than someone you already have, pick up the new guy and sell off the old player (finances permitting). This is actually something that I have been doing all along, and I found for a minimal outlay and marginal increase in weekly salary my bench incrementally got stronger and stronger.
Step 4: Build towards running a 7-man starting rotation, 3/2/3. That's 3 smalls, 2 SF, and 3 bigs. Again, versatility is desired, so if any of the smalls or bigs can play SF that's good, and of course the SFs should be able to fill in at at least 1-2 other positions. The rest of the lineup is covered by the decent bench players you've been improving along the way, and should include at least 2 smalls and 2 bigs.
That's kind of where I'm at now, and finances would prevent me from running any deeper given my current salaries, until I promote to the top division :)
My #1 philosophy for all bench guys would also be that they MUST be able to defend their position! I'll happily sacrifice quite a bit of offensive power on them - it's not their job to score, but their defence must be at least within 2-3 levels of my starters for me to be happy having them on the court.