I don't think you're really forced to find treasure in the middle of combat, so ramping up the difficulty shouldn't be an issue unless you just want to blaze through the game for the sole purpose of hunting them down. Just be sure to observe each and every area thoroughly in-between firefights, and keep in mind that a lot of them have to be shot off ceilings and statues 'n' such so you can reach them, not just lying on the ground or ledges you have to climb to reach like in Drake's Fortune... So, keep your eyes up. Observe the scenery carefully.
I say all that, but I still need to find about 25 more. :| Didn't use a guide for the first game, so I'd like to avoid using one for the second as well.