As a designer, I make sure my seams idealistically fit together best. My pattern testers will correct me if they seem wrong.
As a quilter, I let the fabric do the talking. If it wan't to go to the right when it should be going to the left, I let it. Ultimately, I will end up with a smoother top. Now, I might have an issue with the next seam, but I will still let it do the talking. I follow what it says. Not all fabrics are created equal nor have they all been starched the same.
When I first started quilting, it drove me crazy spending all that time properly lining according to the pattern only to end up with a bumpy mess or cut off points. Maybe it's just me? Eventually I just went along with fabrics request. Quilting life became easier when I didn't argue with it!
If you're having a hard time fitting those pieces together based on a pattern, just see how it wants to fit together. Iron it in that direction then pin. Make sure you follow the quarter inch seam allowance. Iron open and it should be good to go.
Hope this helps!