Lol....I want to say it was Gen. MacArthur who said "no generalization is worth a damn....and that includes this one."
Yes Mushroom, sports are the exception to the 'vegetative propagation breeds true' rule, though I am not aware of any data that indicates tissue culture causes sports. Sports....which, btw, result from mutation in cells of the rapidly dividing apical meristem, are enough of a rarity, that they would not affect the nature of the 'Truly Named' program. And as Sean pointed out, a goodly number of sports revert to normal. The stability of a sport depends on the proximity of the mutated cell to the hub of the apical meristem. The closer to the meristem, the more rapid the division, the greater number of chimeral cells produced...the more stable the mutation.
And since we are looking at exceptions, there are a handful of plant hybrids that have been back breed for so many generation ( fifty+ comes to mind) that they now breed true to type, so technically it is inaccurate to say the hybrid "always" breaks down in the F2 generation, but since none of the back bred plants I am aware of is a Nymphaea....the hybrid break down can be considered to be fact in those discussions.
In essence Greenthumb...maybe.<g> Species are pretty much exempt by definition from concerns over sexual reproduction. One defintion of a species is that they they do indeed breed true to type and that is because they share the same genome. Hybrids by contrast are the progeny of sexual crosses between members of different species...so while in a species X species cross there is but one species involved as parents, hybrid crosses involve multiple species and there is no way of knowing which traits from the various parents will be expressed. So even if an offspring resembles a parent....which always baffles me when it is said because it by definition has more than one parent...it cannot be said to be true to type. But here again, that is not a function of the Truly Named program, it is part and parcel of the International Code of Nomenclature for Cultivated Plants (ICNCP). Truly Named plants just abide by the tenets of that code.