I have used FXCM and the defaults because for me it seems to match the real costs. The big problem is that spreads, swaps, ... change over time. For better backtests it is essential to have ask and bid.

You have mentioned that you use Oanda, which was a good broker when all the others were not so good. I have traded with Oanda in 2012 until I found out that the performce of my system was 10% and more less than the results with ActivTrades, LMAX, ... . During the European session ActivTrades has a nearly fixed spread of 0.7 pip for the eurusd and they don't have more slippage than others.

What I really don't understand WHY I should fix the asset info manually when everything(except the symbolname) is available via MT4?