In general it is best to use and generate strategies on the data the strategies are going to be traded on. That means, for the best result, you should trade the strategies on Dukascopy because that is where the free Premium data feed comes from. The next best thing would be to attempt to use the commission, swap and spread information from your actual broker. I *always* overestimate my commission by using USD$10 (basically the maximum a broker will have), and double current spread (double from when I record the spread). Even if I am using the same data for trading and generation I still overestimate the spread. Overestimating the commission and spread will help in real trading, and also if you do copy trading by giving you some extra breathing room because of possible slippage, execution delay from sender > receiver, missed trades etc.
The most important thing however is using data from the broker you are going to trade the strategies on. This will have the greatest impact on how the strategies will behave when you put them on your actual broker on a demo or live account.
You can get a demo account with JFD (for free) and as along as you place one trade every 30 days the account won't expire. You might also consider getting an actual account with Dukascopy. If you have *any* live brokerage account with Dukascopy, MT4, MT5, CTrader etc, support can set your demo account for *any* platform to not expire. The latter will be the best option to give you the best results.