That's absolute tosh. You can absolutely take up a pair of jeans before soaking them - especially if you're a 28".
The manufacturer/shop should be able to advise you on how much shrinkage there'll be. Every time I've purchased a pair of jeans from SOAS, they have said: 'these jeans will lose an inch and a half (for example) on soaking'. So we measured my desired fold height etc, they then altered them, I took them home and lo and behold, they came up by an inch and a half - resulting in my desired fit.