Is it a good or bad idea to paint walls and trim the same color?

Same color is perfectly fine, just use a different finish. I use gloss plant on trim as it is easier for day to day cleaning.

Well, I wouldn't go for an exact match because some contrast helps the trim stand out a bit and look nice. I would at least go for different finishes if you like the same color the best.

Personal choice. Do you plan to paint baseboards, door, and window trim same color? What about the doors and windows themselves?

Like Naomie said, you want a gloss for the trim to make it easy to clean. But gloss on the walls would be glaring and show every slight imperfection. If you do the same color with different finishes, the color will look slightly different. And like William said, it's a personal choice. Painting the trim a different color gives separation and frames everything (doors, windows, etc).

It definitely is a personal choice but keep in mind it's just paint so it's easily changed. I've seen lots of of places that paint everything the same color, trim, walls, ceiling, and it looks fantastic. It depends on the look you're going for.

I love the look of everything painted the same color. If you look at magazines like House Beautiful, most of the homes are painted like this. It's also a good way to make your room look larger because nothing stops your eyes.

Home owners preference.

Definitely depends on the look you're going for. There are some really cute rooms with the trim done in a contrasting colour, and others that look amazing with everything the same. As someone above mentioned, it can make a small room appear a bit larger with the walls and trim matching. Ultimately, it comes down to personal preference and your desired look for the room.