Save any rare you need for a fusion (specifically Rhazin). Some say keep 1 of each rare just in case. I lack patience, and probably "ate" many rhazin pieces. I regret that now.
Other advice:
1) dont use epic tomes on bad/average epics
2) farm, farm, farm
3) be wise with silver, dont upgrade crappy gear.
There are some good rares, and they're easier to max out using tomes than epics and legendaries.
Apothecary and Coldheart are comparable to epics in their abilities (some say comparable to legendaries), You'll see people use them even in the end game because they're still good champions. Don't look at the rarity, look at how they work.