When I'm not scalping ORB's or trading within the first 30 min, I like to have a starter position, then get one add if it sets up. For example: I buy 1 TSLA put at VWAP test/fail. TSLA then sells off some more, gets to LOD, then retraces to just above the 34, until it shows weakness. That's usually where I add. I only add to winners. Used to short stocks really extended off the 8ema, at resistance, and if/when they got more extended, I would add to position... 1 x 1 x 2 x 2 x 4. This strategy went really well but one day a trader I admired had a bad trade and lost over $20k. That day I switched back to trading long options. He was probably averaging $500-$700 a day minimum, and all it took was one bad trade. (stock was EXAS btw)
I think it depends on the specific trade and your style. For a perfect swing if I take a great entry, and keep a very tight but realistic stop loss, with a realistic take profit... I'll go all in, and all out. In fact I'll have stops and limits and not touch it at all. For a 4 dollar pump and dump I go full size entry, and then usually scale out. Ie if I'm lucky to be in for a parabolic move I have to take 50% when it's up like 20% in 5minutes - even if it still looks strong. Leave a bit in there to ride and see where it goes. I caught two of these just last week, and thankfully I left 30% in to ride for another day. 60% trades they ended up.
You can choose to go all in or not. There is no middle ground to it, if you ask me. Every trader should be aware of when to enter and exit.