With extract recipes, as long as you end up with the right amount of wort in the fermenter its almost impossible to not hit the OG that the recipe says it should be. With extract recipes the sugar conversion is already done so you don't have to worry about it much.
Also in terms of Partial Mashes and even all grain brewing, your recipe will have an approximate starting gravity based on your recipe, you may not have hit your numbers, but at least it will give you a ball park idea of what your beer should be at.
Add it to your brewing check list so you don't forget...I always print a check list even though I've done many batches....it's a good double check for those brain farts