yes, you can. i'd highly recommend it...
From TWS user's guide....
"Percent
The Percent option was created as a safety net to prevent you from transmitting a limit order that has a mistyped limit price. Any limit order with a price outside of this calculated percent off the market price will be considered erroneous, and will not be transmitted. The market price used is the price displayed in either the bid (sell) or ask (buy) field on TWS at the time you transmit the order. This option can be left blank.
Press Alt + S, then P"