Amongst older GB stamps there are some stamps that have very fugitive ink (water soluble ink)
And not just GB; there are a whole lot of Commonwealth stamps and others on chalky paper that can lose the printing.
Austrian stamps had varnish lines under the printing and the varnish can dissolve. This is actually what it was intended to do to prevent washing off postmarks and re-use of stamps
There were lots of experimental colours used as well, mainly pinks and purples seem to be prone to this - I have found some bad ones when trying to soak or clean stamps from Brasil where virtually the whole design can vanish before your horrified eyes!
Some (but not all of these) are noted in catalogues or you can do a bit of internet research before you take the plunge (pun intended)
I also do add a drop of detergent to the water very often as it will improve the final appearance of the stamps and does not seem to have a bad effect on colours (except those I mention above - they still vanish if they're going to)
I have a couple of Desert Magic drying books which make it easy to flatten stamps. Each book will hold hundreds of stamps (depending on the size) They work on the same principal as what Rod has described, but they're already made for you. There's an impervious page to put the stamps on (even if you don't get all the glue off they will ping off if you bend the page) interleaved with thick blotting paper to do the drying.
I find you can take the stamps out after about twelve hours or so and they're generally nice and flat, but even if they want to curl a bit you can put them in an envelope and they'll be flat next time you look. Before I put the blotting paper page on top of them I dry them with kitchen towel so they're pretty dry before I close the book and put them aside
One last issue with soaking - some stamps for example Austria between about 1890 and 1910 and some stamps from Iran - can roll up into little tubes which are very tricky to flatten out! Infuriatingly it's very hard to work out which ones are going to do it and it is only some papers, not the whole lot.