Again, think of it in terms of what's "not there" is not drag. Look at all the fastest ships on the planet, what do their wing tips look like? High taper, no or little rounded tips. A good example of just a straight flat is this: http://www.aerospaceweb.org/aircraft/research/x15/

The smaller your wing area, the faster you are going to need to be going to create enough lift to stay in the air. I custom build a turbine F-20, and without it's wingrockets, it is a straight edged flat tip:

http://www.fas.org/man/dod-101/sys/ac/f-20_3view.gif