Every large collection has embarrasing pieces that were mounted in early years. They serve as reminders of how far a person has come.
This is not to say that you should not mount common things. If you have a good quartz or calcite, but all means make a mount of it.
Another guideline I follow is to mount big pieces. I like to see some context, matrix, and associated minerals. Some people like to mount individual crystals. To each his own, but this has never appealed to me. Some people seem to do it as a display of their skill or something of the sort. When I have had opportunity to be selecting pieces for purchase from a collection that is up for sale, I won't even bother to examine tiny pieces or single crystals mounted in a box.
Here is another way to look at it. Don't mount things that look like they fell off of a bigger specimen, or material with damage that looks like it was debris in the bottom of a box. Mount pieces that look complete and entire. I pretty much try to fill the box, but there is something about mounting a piece where the crystals and matrix look "balanced". This is something to strive for, though clearly not all available material will allow it.
Another guideline when purchasing is not to just buy a label. If I cannot find a recognizable and/or interesting crystal of the mineral claimed by the label, I pass it by. I extend this thinking to rare minerals in general. I won't don't want to add a colored smear or ugly crust to my collection just because it is a rare mineral. Could this rule be extended to be "no ugly minerals"? Perhaps. I will make exceptions any time something interests me for whatever reason -- but my goal is never to "be the first on my block to collect them all". In my view this leads to a mindless "collector mentality" that I want to avoid.
It takes valuable time and energy to make a micromount, and rejecting mediocre material is a way to focus one's time and energy where it is most worthwhile.
This is not to say you shouldn't mount more than one specimen from a given location. And you don't have to make an excuse to mount more than one, such as finding unique associated minerals. If you really like it, mount it up! Mount as many as you want. I know one fellow who mounted over 20 pieces of Carminite from one location where he collected.
I also know a fellow who makes it a strict rule to mount just one and to call it done. The wisdom in this is recognizing that each mount is work. If you have a lot of material waiting to get mounted, time saved can be spent mounting some of the waiting material instead of making duplicate mounts.
But in the final analysis, it is your collection. If you are having fun mounting 10 or 20 Carminite or Chlorargyrite specimens from a location where you hit it rich, you should go for it.
Tom's Mineralogy Info / tom@mmto.org