Yup...I tried this too (riding a geared bike and not shifting) and wound up picking a gear ratio that is too easy..fortunately I realized this before I actually got my first singlespeed. The trouble is with trying this is that you tend to think about what gear is best for climbing, forgetting that most of the time you will be spinning on a nearly level surface. The temptation to shift throws your judgement off you end up picking a gear thats lower than you'd really want if you were actually stuck with one gear.