The below post is about risking losing cards and not so much so about your question. I just got to writing and couldn't stop myself. I am sorry if I am a bit off topic:
We play about 10-15 games a week, and I see a lot of cards lost unnecessarily to a Roll = 7. The below strategies are nothing special -- you probably already know them and I am wasting your time -- but I post them here because some new players don't think of them until after they've lost a lot of cards, and possibly a lot of games:
Trading for what you already have: If you have a fist full of cards but no one wants to trade you that one resource you need to build/buy what you need, try to trade for a resource you already have plenty of. Then, once you have enough, you can trade 4:1 (or less, if you have a 3:1 Port or 2:1 Special Port for that particular resource) and get what you need. It might seem like a waste -- trading with the bank -- but losing them to the Robber is a waste, too.
Don't be so particular: When you can't trade for the resources you WANT, trade for what you can GET. Sometimes it's better to buy a Development Card or Road and forgo waiting for all the resources to build a Settlement or a City -- just an example, of course -- and potentially lose cards to a Roll = 7. This is especially important if YOU are the ONLY player with a particular resource. For example, early in the game, players are often desperate to build roads and scrambling to get bricks and wood. (Yeah, yeah, it's called lumber in the rules, but we are a mono-syllabic group of Neolithic cave dwellers.) If you are sitting there with 3 cards, 1 a brick and 2 wood, guess who's gonna get robbed when a 7 is rolled or a Knight is played? Yeah, YOU!
Take a Chance: This last "strategy" conflicts with the first two, but sometimes it is the way to go: TAKE A CHANCE. There are three factors (that come to me at this caffeine-deprived moment) that can affect whether you should just chance losing cards to the Robber ... but before I talk about them, keep in mind these numbers:
>>> The more players playing, the greater a chance a 7 will be rolled before it's your turn. Every time the dice are rolled, you have a 1 in 6 chance of seeing 7. 6 players? 6 rolls, 6 chances! That's 100%. (Of course, we're talking "odds", not fact.) Now, each time someone DOESN'T roll a 6 and the dice are passed, the odds that a 7 will be rolled before your turn drop: 5 in 6 (83%), 4 in 6 (67%), 3 in 6 (50%), 2 in 6 (33%), and finally your own roll, 1 in 6 (17%).
So, those three factors:
1. How important are the resources in your hand? If you just can't see a way to turn the junk in your hand into something worthwhile, risk them. If no one else wants them, and you can't trade them for something you can use, who cares if you lose them?
2. How many rolls are left before it's your turn? As I explained in the numbers above, the longer you have to wait for your turn to arrive, the greater the odds you may get robbed. Predicting a 7 is like predicting an earthquake; but I'm an "odds" kind of guy, so often I will risk cards rather than buy something I really don't need. Remember, if there's a 17% chance a 7 WILL be rolled, there's an 83% chance is WON'T!
3. What are the odds of getting what you want? If you only need 1 wood, for example, to win the game, and you have 2 settlements/cities on forest hexes with good numbers, the odds that you will get a wood on the next roll might be higher than the odds of a seven being rolled. Take the chance! (Example: if you have a 6 on forest and an 8 on forest, the odds that you will get a wood on the next turn is 10 in 36, where as the odds of a 7 being rolled is only 6 in 36.)
But it all comes down to the board, what you need now, and how close you are to either getting what you want and/or winning. Every situation is unique, so take my advice or ignore it ... and have fun!!