Greetings. I was a corp bond trader in my previous life so would offer a couple generic insights. First, there are two basic and parallel trading markets to the same corp (or muni) bond: The institutional market and the retail market. The round-lot institutional investment-grade (IG) market trades with a 5bp bid/ask and traders usually cut that down to 2-3bps for large trades north of 5mm. The institutional market may be over-the-counter but the market action due to the large Wall St. desks on the sell-side vs. the large asset managers, insurance guys, etc. on the buy-side gives it the feel and liquidity of exchange trading. A trader can typically push 2 buttons to get competing quotes and execute a trade within 15 seconds.
However, the retail market for corps is much less liquid and feels like what it is: over-the-counter trading. Even for common IG bonds I would see retail trades that were well off the institutional levels. This, though, is to be expected when you take the thought to the extreme: Since most bonds trade at $1000 par the liquidity for Pimco to trade 50mm WMT '14 vs you the retail dude trading 5k of the same bond will prob be off substantially. The back-office costs, broker costs, etc. for trading a bond does not increase or decrease proportional to the size of the trade in theory so $300 (cost example) for trading 5k of bonds vs. 50mm of a bond makes the ultimate price to the trader vary greatly for the same bond.
Trace data will then vary widely for the same bond. For you, it is prob best to get a feel for the price difference for your bonds between large trades and small trades that are close together - that difference will give you an idea of what price to expect for your retail trade vs the institutional price that you may see quoted or on Trace at the same time. The bond market moves second to second so looking at prices an hour apart will tell you very little about the reason for the actual price difference.
Finally, I suspect that you have to use Fidelity to execute your trades so prob worth looking into what their costs are vs other retail brokers. My sense was that Fidelity prob did fair execution for their retail accounts but I can not confirm that - Fidelity's execution of markets vs their total transactions costs are two different things as well.
Good luck.