I have generally found the volatility at MtGox to cause issues with intervals less than 1 hour.  Most of the time I run it at 2 hours, and FYI I use a 9/17 EMA split instead of a 10/21 in order to make the trend lines a bit more sensitive at that interval.  I've seen some others use a 7/15, but I haven't really tried that.

Also, as Pablo points out:  using EMA trading, about 70% of your trades will be small losses.  It's the 30% of large-gain trades that will compensate you in the long term -- and EMA trading is a long-term strategy. 

There are ways to minimize the small losses if you're thoughtful, keep on top of the market data, and adjust your settings once in a while.  For the last two days or so, for example, the MtGox market has been VERY sideways (the EMA diffs and price fluctuations are small, and the trendlines *slightly* cross over and over again; also, the trading volumes are low).  At times like this, I generally like to pause the bot and hold on to my Bitcoin; when the volumes pick up again (likely Sunday night, when China starts a new week), I will unpause it and let the bot take over.  Another option would be to temporarily increase the buy/sell thresholds to keep the bot from trading.