Thing is, a lot of lower tech worlds will be sending bodies into the military, too... And those folk are likely to balk about vat-meat and artificial veggies... as much as the hive-worlders will balk at eggs with, "That came out of a chicken's butt? Ewww...."
I can imagine the Imperium taking several measure with regards to rations. The first is one we see already; developing an "institutional cuisine" which is (hopefully) the least objectionable to (hopefully) the most number of people.
Second is another measure we already see; taking care to ensure "cultural treats" are available on certain occasions. I'll point to the great effort made every year to provide turkey dinners around a certain Thursday in November. Another example was my ship providing a steak and lobster tail dinner once a month for each crew member whose birthday occurred that month.
Finally, "boot camp" may automatically involve specific training with regards to food; i.e. Eat what's in front of you/Hunger is the best relish", as much as it automatically involves Gun Combat-1 for the army/marines, Pilot-1 for the scouts, and so forth.