Contrary to what the wiki says, modules do not necessarily turn yellow when they hit 50% HP, as detailed in http://forum.wotlabs.net/index.php?/topic/1566-for-questions-about-crew-skills-equipment-and-modules/&do=findComment&comment=454376. What actually happens is that modules have a base HP amount and a natural regen point. So long as the module does not go below this regen point it will not appear damaged, once you pass that point it becomes yellow.
This regen point isn't the same for all modules, either. Ammo racks seem to hover around 60-70% of the normal max HP. The Type 59 has a regen point of 100 HP, and a max HP of 170 (58.8%). The JP E-100 has a regen point of 190 HP and a max of 280 (67.9%). These values seem to vary per tank and will need to be looked up through software like Tank Inspector or WoT Knowledgebase (or tanks.gg, an excellent website). Some regen values are at 50%, possibly the origin for this particular belief.
This narrower-than-expected window means safe stowage may not be as useful as one would otherwise expect. Consequently I've made a spreadsheet (what a shock!). Here you can input the tank's max HP and regen point. In addition to giving you the average, minimum, and maximum module damage rolls of every gun caliber in the game, it will tell you what calibers safe stowage will provide a benefit against. Keep in mind this only matters if a round succeeds in its 27% (or 30% with deadeye) chance to damage the rack in the first place.
Spreadsheet. - Make a copy for yourself and you can check out any tank you desire.
To explain what you're seeing, here I have used the Type 59 as an example vehicle. The sheet is saying that safe stowage protects against...
Ammo rack damage by
Average roll of a 57mm
Minimum rolls of 75-88mm guns
Maximum rolls of 45-50mm guns
Ammo rack destruction by
Average rolls of 128-139.7mm guns
Minimum rolls of 170-182.9mm guns
Maximum rolls of 105 and 107mm guns
So, it's not a stellar perk, but I think in light of the dearth of useful loader-specific skills, it's worthwhile considering it. I should mention that I don't know if the regen point is buffed by safe stowage, but Tank Inspector says it is not and I've no reason to disbelieve it. Wouldn't make much sense for it to be anyway (though when has that ever mattered?).
As an aside, the British 20pdr guns break the mold and have more module damage than you would expect from their caliber, possibly an arbitrary bonus given for balance purposes.
Edit: I was fighting with the Google Sheets conditional formatting rules for a while, let me know if you see something wrong with the setup.
Edit 2: The sheet is written in such a way that you can alter cell H4 to use formula "=H2*1.5" if you want to see how wet ammo rack stacks up instead of safe stowage. I may go back at some point and add it as its own feature later, but for now I really don't want to re-do the conditional formatting rules.
Edit 3: Took the easy way out and added a WAR sheet. Might make work on a combo later.
Edit 4: Combo sheet added.
Edit 5: Updated for 9.18 artillery module damage changes.