If you haven't done this yet, here's my advice (after screwing up several
Mark the needle positions as many different ways as you can, while the engine is completely cold and you have less than a full tank of gas. Take pictures of the needles, draw diagrams, etc. Don't half-ass it, if your needle points just below the L but not exactly on the L, draw it that way so you know.
For the tach, start it up and right down exactly where it sits when you are idling, and when you're done with the install, start it and press it on exactly where it was.
Speedo, once you get it all installed, take a drive next to a buddy and have him yell your speed out at several different speeds, then press the needle on. Then speed up and make sure it's still right.
The needles dont need to be jammed on there hard, just press them on gently, and take your time prying the old ones off (forks work great.)
I guess the real consolation is, if you do screw up the needle positions, they're basically all useless anyway (except the speedo, which is questionable,) since they just point anywhere. Long as you know where the needle usually sits, you can be the judge of whether you have problems or not (my needles are way off, but I know where they normally sit so I know if the charge is too low/high, oil pressure, etc.