I'd say thats probably true specifcally with the tRAS timing, say for example a 2-3-2-6 set ram, if it's lower it takes less time for an entire set of data to loop through the ram stick inbetween each charge, which makes it faster compared to the same stick that has 4-4-4-12 but that 6 set has a higher risk of corrupting your data which leads to the "blue screen of death" every once and while, but in the case of a 12 vs a 15 set, unless your motherboard is horribly picky or horribly old you shouldn't have that much of a problem, if it does, id go with flashing the board or getting a more flexiable one, something else to consider is that even if your timings are perfect for your board it could still lead to instabilty by the level of heat that it produces or how overclocked it's other timings are, like tREF, ive had instances where it will just sit there without any power about 30min into a boot and the ram will just lose power and ill get blue screened, so id say your motherboard would play a bigger role on how stable your ram is more then the timings. too much or too little of something is never good, LOL. ive had machines that run 2-3-3-4 without a hitch, i think it just matters on how beasty your motherboard is.