each has their advantages.
bombing alone means you don't have to worry about anyone slowing you down, you also have more options when it comes to hiding if you did get chased. You also have more options for spots that you want to hit because A) you dont have to share B) you can go as big as you want. On the flip side bombing alone makes you look more suspect. One guy with a backpack walking around by himself is a lot more suspect than 2 or 3 people just walking and "hanging out." The multiple people also allows for look outs which should help you avoid chase in the first place, and people who can boost you up to spots you wouldnt be able to reach solo.
when it comes to painting, multiple people again means more look outs, but also helps when it comes to buffing the wall, doing a background, carrying paint and equipment (i.e. rollers, bucket paint, roller tray, etc.). piecing alone lets you be more free, and you can paint faster since no one else is talking to you or asking you for input/advice. Its harder to hear when someone does come through though since you either have to constantly look over your shoulder or take a gamble and hope no one will come through. Painting alone is probably a bad idea if you have beef with other writers as well, since you have no protection if you get jumped.