Most people do use a sash brush to do their cutting in.
I own a small apartment block, so I'm typically repainting apartments the same colour as they were before. If you're doing that, you can cheat by using a piece of sheet metal and a 3 or 7 inch paint roller to do your cutting in.
Just apply two widths of 3 inch painter's masking tape to one edge of a two foot long by 18 inch wide (or so) piece of galvanized sheet metal. Slightly bend the sheet metal into the corner to protect the side you don't want to get paint on, and spread your paint up to the sheet metal with the roller.
Don't start with a fully loaded paint roller near the sheet metal because if the roller is too full, it'll squeeze paint under the sheet metal so that it comes oozing out on the other side, getting paint all over the side you didn't want to get paint on. Load up your roller and start rolling about 12 to 24 inches away from the sheet metal and spread your paint up to the sheet metal.
And, when the painter's masking tape on the business end of your sheet metal gets all gunked up with partially dried paint, just pull the old tape off and put new tape on, and you're good to go again.
Don't buy regular masking tape because the glue on it can be too aggressive and pull the paint off the wall when you remove it. Buy painter's masking tape and press it down tightly on the edge immediately before painting over it.
Where I live, painter's masking tape comes in different widths, but it's always 55 meters in lenth, or 60 yards per roll. A meter is about 39 1/4 inches.