It depends... using a ton of it will change the character of your beer, but using a relative little bit to get your gravity will pretty much do just that, raise your gravity. It's personal preference - some say leave lower gravity and just accept the alcohol where it ends up. Personally, I always over buy DME for my starters and I have it on hand to adjust if I miss my gravity from a mash. Leaving the beer ten points low will change the character too - for me, might as well get the gravity you intended to. This is exactly why I check my pre-boil gravity right after the mash from a cooled sample. If it is low, I have plenty of time to calculate some DME to put in at the end of the boil. say you have 6.5 gallons of 10.42 - that's 6.5 x 42 = 273 points of sugar. You were supposed to have 6.5 x 53 = 345 points of sugar (10.53 OG). So you are 72 points short. DME adds 46 points per lb, so... about 1.5lbs of DME should get you the 70 or so points you need.