1) It is definitely possible to overbuild, and for overbuilding to be unsafe. As was mentioned, every time you add weight, you need to add more strength to carry the additional load - which usually means even more weight. It's a vicious cycle. Overbuilding costs you money (materials, plus you need to buy bigger motors and parachutes). All else equal, a heavy indestructible rocket is going to cause more damage when it hits the ground in the most common failure scenarios (lawn darts, tangled chutes, etc.)
2) You're only as strong as your weakest part. No sense getting a 1000lb rated forged eyebolt if it's going to be installed in a cardboard rocket on a lite ply bulkhead - any force large enough to open a standard eyebolt is pretty likely going to severely damage the other components. A lot of overbuilding seems to come from an attitude of "if a little is good, lots is better!" when in reality you could just be adding non functional weight. E.g. huge internal epoxy fillets when a modest amount of wood glue already creates a stronger bond than the fin itself. Even in composite bonding, there's basically a max useful bonding area - extra epoxy is just going to get squeezed out and/or be essentially non load bearing ballast.
3) Unless you're going high mach or using super high thrust motors, flight itself probably won't be the nastiest force on your rocket - either landing or deployment shock, both of which may be greater loads at off-axis angles, will probably be the worst and most damaging events. Again, overbuilding might be bad here, since more weight will put more loads on your recovery system at deployment.
4) As a result of 3, it might make sense to "overbuild" for flight loads in a few key areas, more for durability than flight safety. For example, if you fly a wet field, fiberglass might be desirable just to avoid a soggy rocket even if you don't need the strength. If you fly over a hard surface, more advanced fin reinforcement techniques might make sense to prevent landing damage. Or maybe you just tend to get a lot of "hangar rash" so you use stronger fins and more durable finishes.
5) Epoxy actually has a lot of good qualities you might like even if it's not the "best" adhesive for your application. It makes nice aesthetic fillets because it doesn't shrink. Quick-cure versions give you more working time than super glue but don't take nearly as long to set as wood glue. It doesn't bind up when sliding couplers or centering rings into a tube.
6) As others have said, build for what you want to do. Do you want a tank that will survive a hundred flights, even if you have a few mishaps? Build strong. Do you want a good performer? Build efficient. But whichever you do, build smart - if you want to deviate from the instructions, ask yourself "what am I adding by doing this?" Increasing durability for off-nominal cases, reinforcing for a bigger motor, or even ease of construction are all valid answers. "I want the fin can to survive the detonation of a small nuclear device, even if the fins themselves and everything north of the motor tube has been vaporized" is rather less valid.
Short version - I'm with Binder on this one. I've not had a chance to build a Binder to see his instructions, but I will say I've honestly never had a rocket fail during flight when built according to kit instructions due to lack of strength (and assuming you don't wildly exceed the motor recommendations). Even the much maligned Estes "tea bag" shock cord mount has served me well on LPR and PSII rockets (I've burned through shock cords with much use, but never ripped out a tea bag). All my rocket damage has occurred in off-nominal events (chute fails to open, CATO, delay was poorly timed, etc.)