No matter how hard you try, rear subwoofers will usually "reveal" their location with certain types of music, but this won't happen with the subs up front. The ultimate setup is to actually have the subs there. Many of the higher competition classes have examples of this, with subs in doors, dashes, floors, and kick panels. This is definitely an "extreme" approach, but it is also the best. Subs in front can really make the sound stage realistic, since all of the audible frequencies are coming from the same place. Also, woofers in front of you can be crossed over higher, taking some pressure off of the midbass drivers.
Needless to say, it often takes a lot of custom labor to install large subwoofers in the front of the car. I've seen everything from 15" woofers in kick panels to 12" subs in dashes, and while they may work very well, it's usually not necessary to go to all that trouble. A single 8" or 10" sub in the proper enclosure is all you need for a competition system, since it's not necessary to hit extremely high volume in the judging lanes. If your system can play 110-112 dB cleanly, it's plenty loud for sound quality judging. If you are concerned about hitting higher SPL, switch your rear subs on only for the SPL portion of the judging and keep them off for sound quality.
A great location for a front-mounted woofer is the glove box area of your vehicle. If you are willing to give up the glove box, the space created can usually accommodate an 8" or 10" subwoofer (requiring a volume of 0.3-0.5 cubic feet). An enclosure can be created without altering the rest of the dash and, if you are careful, the car can easily be returned to stock if need be. Don't worry about the offset location of the speaker, since it won't be audible when it's set up correctly. In my experience, subwoofers in this location also "load" very well into the car, so they will have smooth frequency response and low extension. Be sure to choose a subwoofer that can play low, since many competition discs (like IASCA) have extremely low bass frequencies below 20 hertz that must be reproduced accurately to score maximum points.
When you have a system with up-front bass, you will be amazed how much more realistic it will sound. Now get out there and do it and I'll see you in the lanes.