You can, but it's inadvisable in my experience.

The thing to be aware of there is that the bid/ask of the combo is not necessarily the combined bid/ask of the individual legs. IB will factor in "native" orders for that exact combo into the price calculation (including your own orders if you have any in). So for instance, if there are a couple of orders in the book for your exact straddle, the midprice quote for the spread will differ from the combined mid of the legs. This can result in you mispricing your orders if you're not careful.

This drove me crazy (and wasted some cash) until I clued into what they're doing.