Any backtesting strategyb should simulate reality as closely as possible in order to avoid nasty surprises when you go live with your strategy. Using ask prices for long entries and bid prices for short entries would help improve simulation accuracy. Of course, as mentioned above the bigger the spread and the lower liquidity - the more important it becomes to use bid&ask.
The answer will also depend on what frequency your live strategy will be generating signals based in production - every second, every tick, X-min or X-hour bar close prices only? If you backtest on hourly bars using close prices, but in production your signals are generated based on 1-second price checks - your live trading signals maybe be very different from backtests.