When you go to sew two blocks together I always tack the corners together but I bend back one corner so I can make sure the seams match on the inside. Then I tack on the side that your seam will go through. I had a student that tacked on the top and then couldn't figure out how to open the seam to iron it. I always open my seams to iron because open seams are less bulky than ironing to the dark. That is more important in minis than in a reg. size quilt.
When sewing together two sets of half square triangle blocks I tack in the center where the seams meet and then I tack at the end where the corners meet. I rarely use pins for this because I feel the pins don't hold it as firmly as tacking and everytime I choose to cheat and sew without tacking or use pins instead -about 75% of the time I have to take it back out and then I am kicking myself for trying to hurry the process.
I HATE to take out seams of any size!!!
This attention to detail really pays off most of the time. I will still have a problem if the corner of the block wasn't right to begin with. If my poin tis off I go back and take the corner apart and look to see if the seam is even in the middle or if one side or the other is bigger than it should be. I will trim it gently and try again. Sometimes if your seam is not right you can just sew along the seam and make a tighter seam Sew on the inside to make the corner more even.
I just did this yesterday. I had one that I had two blocks sewn together and was putting two sets together. The problem was that one of the two blocks in one set was sitting at an angle instead of being square with the block. I ran another seam along the existing seam and came in about an 1/8" or less to make the block strtaighten up and be squared off against the other block. It's kinda like sewing a tuck into a blouse - you start at the original seam and then angle your newer seam in.
If this doesn't make sense I can show you pictures I think I diagramed this for something I did.
Sorry to be so wordy!!