It is safe to set spread a little bit higher than the average real spread.
Please note that , if you set Spread = 0 in the "Data export" script, it will use the current spread at the moment of the export (rounded up to a prettier higher number ).
Plea be careful when testing with the MT's Tester because it works strange when you make several tests with different spreads. It may be only on my machine but I found it very unreliable.
The best practice is to always use spread, swaps and commissions that are same or higher than the real ones.