18 Gauge speaker wire ought to be adequate for your stereo unless you're running amplifiers that have greater than 25W/channel RMS. If you're only using the head unit to power your speakers (head unit = CD player, radio in the dash) 18 gauge should suffice.
The time(s) to go with heavier speaker wire are when you build what I'm getting ready to modernise...I currently run a Rockford-Fosgate 4 channel amp (60w/channel RMS) along with a 400W subwoofer amp.I use 100% copper wire...10 gauge speaker wire to the 4 standard speakers, MONSTER cable to sub...100% copper 0 gauge amp power wire to junction block, 2 gauge copper wire to amps from block, 0 gauge grounds. I'm going to be modernizing my sound system and adding at least one more channel, with my intention of adding 6.5" component speakers with the mid bass driver going in the lower door panels & the tweeter likely going into a pod mounted somewhere on the A pillars as the 3.5" front dash speakers add very little to the overall sound while severely restricting the amount of power I can use from high end amps. I *MIGHT* also add 6.5" component speakers to the rear side panels aside each passenger. I currently have a set of Infinity Kappa series 6x9 speakers in the rear deck (with adapters). I can't make this decision until I hear how it will sound. I also will be updating my head unit to a bluetooth unit that charges cell phones etc etc.
For me I HAVE TO make 100% certain I NEVER have to suffer...sitting in traffic with someone cranking (C)rap so loud my body vibrates in my own car. I literally want to murder people the instant I hear rap/hiphop/techno/industrial etc etc and I do enjoy the look on their faces when they can hear the 'Stones emanating from my car.... the looks on their faces are quite priceless.