Monopoly Deal: My Bank Pile - Private or Public Information?
The "What The Game Looks Like" info graphic in the Monopoly Deal rulebook strongly implies that the bank pile is public information - the bank piles in the picture are quite carefully arranged so that each card is partially visible (in much the same way as Magic the Gathering players conventionally arrange their graveyards).

With that said, "strongly implied" is not the same as "explicitly stated".  If everyone in your playgroup enjoys the new element of strategy that playing in this way adds, then I say definitely go for it.  As you say, you trust your group not to cheat, so the potential major cheating problem you envisage is not a problem at all.  If there is such a thing as high-stakes Monopoly Deal tournaments, I imagine those would have to enforce the keeping of bank piles as public information.

In the end you just have to decide which of two different play styles your group wishes to subscribe to.  With hidden information, some very exciting play scenarios are created, where with a good poker face you may be able to trick people into believing you're not doing as well as you are.  With public information, you create a much more calculated game, where players have to find an objectively best strategy in the face of all dangers.  Do you consider yourself more of a poker player, or a chess player, that's the question?

Personally I'm not generally a big fan of concealed information for its own sake.  Having to count cards, watch other players' movements and demeanour carefully, and keep track of information in my head - that can turn into a lot of work for the player who wants to do well!  But of course your mileage may and probably does vary.  If the game seems boring to you when all the cards are on the table then you aren't wrong to want to spice things up with a house rule, and you almost certainly should do so!