Seams
 

I am working on a diaper bag. Should I trim the lining seams before sewing it to the bag. The instructions doesn't indicate that only after I sew it to the bag.

It's likely to be laundered occasionally so I would treat it like a garment and zigzag the seams so that it holds up in the washing machine. Only takes a minute to do so

If you can serge the pieces together as whole blocks before assembling the bag, the seams are already finished. Even the lining and any batting you might use for fullness will be "tamed."

I wouldn't trim any seams..diaper bags get alot of rough use...seams hold together better not trimmed.