By the time I get around to moving everything to 3.3v, will the majority of boards be perhaps 2.3141592653v ?
No, the next move will likely be 1.8V or 1.2V, but thus far I've only seen full-sized processors and FPGAs use those. (I have an Odroid HC-1 SBC, which has a 1.8V UART, directly connected to the Samsung Exynos5422 octa-core A15/A7 big.LITTLE CPU. I was using 74LVC1T45s, but will shift to TXU0202s for level shifting to that UART. Darn thing isn't even 3.3V-tolerant!)
(Besides, 3.3V×3.3V/5V = 2.178V, not approximately π - 1V.

)
All my stuff is 5v.
Ouch. Better start stocking up on level shifters, or 5V tolerant transceivers.
For unidirectional stuff with direction selection, I can warmly recommend 74LVC1T45/74LVC2T45/74LVC8T245. For fixed-direction unidirectional stuff like UARTs, SPI, and so on, TXU0n0m. TXB series is very widely used for I2C. For inter-board communications – like when I'm driving stepper driver controller boards – I do love using isolators instead, because that lets me use a separate grounds for the digital logic and motors; especially the Si86xx chips, as they only need 0.1µF supply bypass caps and optional series resistors for noise immunity, so even an uncle bumblefuck hobbyist like myself can work with them easily.