In my aircraft, the muffler or a sideways mounted engine is the single largest contributor to latteral imbalance.

I use plumbing solder to balance the plane. I tape bits on the wing until it balances, then I drill holes in the wingtip to fit those bits, and CA'em in. A bit of covering patches over the weights. I would stay away from placing them in the leading edge, this is more heavily loaded structure than the tips themselves.

If you don't have solid tip blocks, you can open the tip covering and CA the pieces directly to the wingtip framing, I guess. I typically build solid tips.

All of my planes have required a little bit of latteral balance, except the new foam winged gasser I built. Wood varies in weight, and glow engines have that huge weight on the side to keep'em quiet.

The biggest effect I've seen from poor lateral balance is a plane that's trimmed for straight'n'level will roll towards the heavy side in high-g manouvres (loops, pull-outs, etc.). Aileron trim within reasonable amounts of lateral imbalance isn't going to put much differential drag into the airframe, not enough to yaw the plane visibly. That's more likely due to improper thrust line setup, or bad rudder trim, or the vert stab is not straight.

Also, adverse yaw need never be a problem. Simply build differential into your aileron linkages. I find a 3:2 or 2:1 ratio between aileron up and aileron down works well on most models. It's easy to do. More up than down. It's achieved by having the servo horn pointed towards the surface a tad when the aileron is neutral. In other words, the angle between the pushrod and the servo horn is more than 90degrees. Maybe 100 degrees or so.

J