....I have always used a 1/4 inch seam for flannel...I might consider using a wider seam if the flannel I was using was very loosely woven as it would more likely ravel....and yes...just using a wider seam does usually affect the pattern you end up with greatly...unless each and every piece is exactly the same size and shape...then using a wider seam will only affect the finished size as it will finish smaller....for example if you were doing a simple nine patch block with a plain square every other block...this is what would happen....you would piece 9 2.5 inch squares together with a quarter inch seam....this would give you an unfinished nine patch that measures 6.5 inches square and you would alternate that with a plain 6.5 inch square. Now if you decide to use a half inch seam instead....your nine patch would measure unfinished 5.5 inches square so you would have to use a 5.5 inch square plain to have it matching in sizes....so to re-calculate what size to cut all your pieces....you would have to know the "finished" size of them and then recalculate by adding the .5 inch seam allowance.....so if you wanted to use the original 6.5 inch square plain block, you would have to increase your 2.5 inch unfinished squares to be 3 inch unfinished if you are going to use a half inch seam allowance . probably not very clear....but I learned the hard way, if you are following a pattern that has different sized pieces in it...yep...you have to use a 1/4 inch seam or else the pieces don't fit together....