commission is 0

 不過我發現 Fidelity 一般能比Etrade 拿到稍微competitive一點的 bid  和 ask

All these are not important. As i mentioned before, you do not want to day trade bond. For long bond, you want to trade  波段。For example, the 20Y bond I bought around 4.75% in April was sold around 4.1% in late Sept. That was almost 11-12% gain (not counting the 6 month interest on it). 

So, whether you paid for a 4.77% yield or 4.73% yield wont make a huge difference