Unless you are very certain of where you are in relation to high ground or you can pull the chart out and check the MSA (impossible in the circumstance I would suggest) you are taking a big risk by turning. The MSA printed on most nav. logs is only good for 10nm either side of the route and an emergency descent from 350 to 100 is going to put you at least 30-40 miles off track if you turn through 90 degress. I reckon it's better to stay on the planned route unless there is a very good reason to do otherwise, NAT tracks would be a good example.