Joiner callibration tabs

I've been struggling with how best to join together smaller printed pieces into a larger whole. I've settled on having a 10x20x20mm tab-in-slot joint, which makes a very rigid joint. I had trouble sizing the male end to tightly fit the female end, depending on print settings. These tabs let me try various sizes quickly, without printing an entire object.

For convenience, each tab is labled with its dimensions.


Question for the experienced (I'm only two 2 weeks into 3d printing): You can see that the tab labled "18, 8" curls up on the right side. Is that because I don't have a heated print bed?