I wonder when reading tarot cards upside down began. Though it started in the 20th century, I imagine it was akin to any other form of divination where one reads the objects as they lie, such as with throwing sticks, stones or bones. Shuffling certain ways can result in cards being naturally reversed when laid down, so being reticent to "change fate" by altering the arrangement in any way, the cards were read as they were, upright or upside down. Someone, somewhere applied a different meaning to the card if it came up reversed, usually negative. The history of this practice is not likely to be traced, and today we have various theories and practices of reading, or not reading, with reversals.
When I first began reading tarot, the idea of reading reversed cards was daunting. I could barely remember the meanings of all the 78 cards to begin with, much less 78 more meanings for those same cards reversed. It is often recommended that a newbie reader forego reversals until one feels more comfortable with the cards, but that really isn't necessary if you use one understanding of reversed cards: that the energy of the card is "blocked" somehow in the person's life or situation. Basically, the card has the same meaning as upright but the reversed position indicates this is a challenge or obstacle for the person. This is a perfectly acceptable interpretation of reversals and isn't just for new readers only. It's just that it may help a new reader by not adding on additional meanings to the existing ones.
One doesn't have to read with reversals at all. Tarot reading is totally "upside down optional." A lot of it depends on the way you naturally shuffle. I decided to forget about reading reversals when, after noticing that the same cards came up reversed time and time again in my readings, I tracked it to my particular way of shuffling. When I shuffle, I don't get reversals. To get reversals, I had to pointedly cut the deck in thirds, reverse one pile, then reshuffle. On subsequent reshuffles, those reversed cards never got reversed back around. The randomness factor of reversals just wasn't working, so I said the heck with it, turned all the cards upright, and learned more about the duality of the meanings of each card so that reversed cards simply weren't necessary for me.
I can read reversed cards, no problem, but I don't care to lay the cards out with some of them upside down. It's kinda hard to see the pictures that way. Some folks have no problem with it and intuit all kinds of cool stuff from a Hanged Man who is no longer standing on his head but dancing like the World Dancer upright. In general, I am usually able to sense from the interaction of the cards whether or not a particular card is communicating a more positive, negative, or neutral meaning.
As I was discussing a card with my friend the other day, I said, "Well, that's more like the Emperor sideways."
"Emperor sideways?" she asked, "What's sideways?"
"Well," I said, "It's like he's not as dictatorial, abusive and control freakish as he might be upside down, but he's kinda leaning that way."
Oh, don't moan and whine and say, "Oh so now I gotta learn sideways meanings, too???" No, no, you don't. In fact, you don't have to think of it in those terms at all. As you learn to relate the cards to each other and follow the storyline of each spread and throw, you will naturally pick up on how a card is expressing itself in a reading, whether it is to be read more positively or negatively or somewhere in between. If a card is thrown literally upside down, it may limit your perception of its meaning. It doesn't have to, but it may.
There are vigorous proponents and opponents to reversals in the tarot community. Many see them as entirely unnecessary and cumbersome. Others feel they add meaning and depth to a reading and give more information. There are several books by competent tarot professionals that focus on reversals. If you're unsure about whether or not to use them, I would say try them. Read up on the various ways to use them and incorporate them into your readings for a while. Get comfy with them, if you can. Then decide if they are something you want to use or not.
In another post I will demonstrate a reading with reversals so you can see how I might use them, if I did, in a reading. I will also demonstrate how to interpret that same reading with upright cards that includes "reversed" meanings.