The standard 78XX regs have a 10% tolerance, but they have largely been superceded by the 78XXA, which are 5%. If the circuitry can be proven to run correctly at the lower end of the supply voltage then you should probably use a reset chip with a lower threshold. If you have a predictable drop from the regulator to the chip, you could try bumping the regulated output up a bit. You can increase the output a bit if you raise the regulator ground pin in the same way as you would on an adjustable regulator.