Ever hear company called Enron? Do you think each ten bucks down would be a better buy?
Long term stocks I have backtested them 25 plus years, day trading 10 plus years, this is only confident way to know the answers you seek, not asking members. Stocks are not like the ES, they trend till they don't trend, when they go down, trend either neutral or down trend. But really has much to do with your back testing and Trading Plan. I will average down in overall stock position, but each stocks has different percentage down I will do and I use options to add to a position. In stocks I look at averaging down as defensive move unlike scalping ES. But I would recommend just take the loss and find something else. You can always hedge the Long Stock position and do Put Debit spread to help cover possible stock losses, if stock goes up, sell the long Put and keep the short put trying recover losses on long Put.
Enron may be a bad example for they did have at one time a AAA rating on their debt. Each $10 drop probably did seem like a bargain to some people.