Puckering of seams is often considered an indication of shrinkage in a garment.  However, this condition can also be because of shrinkage of the threads or the incorrect thread tension during sewing.  Some puckering may be seen before a garment is cleaned and the agitation of the washing will usually make the condition worse.  If, during your sewing, you see some puckering of seams, even just slightly, be sure to correct the situation right then, rather than attempting to correct it after the garment is finished.  Another thing I learned a long time ago is NEVER buy cheap thread!  In most cases, cheap thread will cause more lint build-up in your machine, will cause thread breakage, as well as tension problems.  Just hold a piece of cheap thread up to the light and you can see all the "fuzz" along the edges.  Stick to good quality thread and you'll have far fewer sewing problems.