Dan, I really like the exchange-supported implied spread markets for anything they are offered in. The legging risk contains slippage variables that can defeat the entire purpose of spread trading as a strategy. The 'challenge' with trading outrights based upon spread correlations - which is a very powerful edge into and unto itself, is that factors unrelated to the spread correlation come into play and can leave a mark. For example, you may correctly believe that the NoB is weak for example, and you may be successful shorting the ten year notes and taking a tic here or a tic there out of it, but if the ES starts to tank or gaps down your short note position will indeed leave a big mark.
nob actually trades 5x3 on the ics if you are a beginning bond trader it wouldn't be a bad idea to restrict yourself to trading on the ics until you get a better idea of how to leg in and out of trades well. i probably trade 35/65 ics/legging ics is very liquid until you need to show 15+ contracts of zbzn and even then you can iceberg so it's not until what your iceberg shows is 15+ that you really need to worry so much about liquidity, and then when the market gets there they sweep you anyways so it really doesn't matter, you just might see them walk up and kiss your offer and run away if you are showing 15+ eh, much safer to trade flys i probably trade 85/15 flys to simple spreads (ie nob but no fyt or zbub) and even then my simple spreads are just overweighted flys (tu/ty/zb or something like that where i'd view the tu/zf as an simple spread and then the ty fly in there)- flys>simple spreads>scalping in my opinion also one thing to be careful of is the spread ratios on the ICS- i wholeheartedly disagree with some of them- for example it is laughable to trade the zt-zf 1x1 and that is what is offered, also if you bought zn-zb and zb-ub putting you in zn-ub, and then did zn-ub ics those give you two different ratios.