Reference tracks are the best resources I've fouund to keep from losing perspective of balances. The impression of a mix can be changed with EQ, compression, and reverb once volume and expression levels are set.
For orchestral libraries such as VSL, I'll use a reference track similar to the kind of ensemble I'm scoring. Sometimes, I can nail the brass sound without any EQ at all. Sometimes the strings just fall into line right away. Other times the winds. Whatever the case, that particular section becomes my point of reference and I don't EQ it, but will adjust the other sections to match, only if needed. I do this as I go, but I don't spend a lot of time tweaking as I go-- until the entire score is completed. Much will be done in post and mastering. I do agree fully that there are no hard and fast rules and that "too much done too soon" often necessitates going back and undoing and redoing later-- or discarding and starting over. The better the balance right at the start, the better the result in advance of processing.
When using an orchestral library, one should consider how the library was recorded, how it was designed to work sonically, and how it translates to your audio monitors without processing first. If I use VSL as the basic orchestral sound, any other orchestral samples from other collections will get EQ'd (and repanned or set to mono as needs dictate) to match VSL. This helps to keep one "toe at home plate", so to speak, in an effort to not lose one's sonic perspective.
Objectivity can also be an indispensible tool. Get a night's sleep before doing the mix, if possible. Get a second or third set of ears towards the end of the mixing process. Keep listening to your favorite CDs again and again to reinforce your sonic orientation and expectation for your own mixes. Doing so makes it so much easier to know what adjustments to make or, more importantly, what adjustments NOT to make.
The other thing about EQ is that it seems to work better as a subtractive tool rather than an additive tool. Cranking the high end may not work as well as ducking mids and lows. I always duck frequencies first, unless it ruins the sound of the instruments in that range. In that case, I know that there are other problems with the mix which should not be solved by manipulating the EQ.
One classic case is that when adding reverb, one can lose the transients of percussion instruments. Many people reach for a fader or an EQ-- when all that might be needed is to add a slight touch of compression just to the timp to make it more present, for example.
