Actually, it is not a rule of thumb about floating seeds, but more an old wives tale. Yes it is true that some seeds that float in water may not be viable, but it is equally true that some seeds that float will be viable (and the same is true for seeds that sink). This will vary among species and floating seeds may mean nothing more than there is a small air bubble adhering to the seed or there is air trapped under the seed coat.
The thing you should be looking for when soaking seeds (presumably after scratching or nicking the seed coat) is if the seeds have imbibed water and increased in size. Those that haven't may need more time in the water or may need more scarification. However, just because a seed has imbibed water doesn't mean that it is viable, it just means it absorbed water.