People sometimes ask me how I get such a nice line when I cut in. There are two reasons for this, the first being experience. When I first started painting I taped everything and no matter what tape you use there will always be bleeding. My dad taught me how to cut in and I have never looked back. The more I painted the better I got at cutting in. I can even do it with either hand. The second reason is my paint brush of choice. The Wooster Softip is my go to. I have used other, more expensive brushes and they don't compare. Don't let the plastic handle fool you, this is a top notch paint brush.
There aren't a lot of products that I swear by but this brush is amazing. Under $10 and it is has just the right softness/stiffness ratio to get a really clean line. It holds the right amount of paint. I use the 2" angle. It's the perfect size for cutting in and it fits in a quart can.
I'm sure this brush isn't perfect for everyone, but have no fear the Wooster Brush Company has so many wonderful products out there. They have been around since the 1800s but continue to be on the forefront for innovation. Check out their website for more products and tips: https://www.woosterbrush.com/