If the S90 rides loose in a bag, the built-in lens cover mainly protects the lens opening, not the rest of the camera. Whether you need more protection depends on what else is in the bag—keys, chargers, and other hard items can scratch or scuff it.

A small case or soft wrap is the safer choice, and compact-camera cases are inexpensive. That said, there’s a real trade-off: any extra layer between you and the camera can slow you down and make you less likely to grab a shot quickly.

A practical middle ground is a very light, quick-access pouch or soft sleeve rather than a bulky case. Some people use a simple padded sleeve or even a bubble-style pouch with the strap left out so the camera can be pulled free fast. Another good idea is an LCD screen protector, since the screen is one of the more vulnerable parts.

So: the lens cover alone is not full protection for bag carry. If your bag contains anything that could rub or knock the camera, use a slim sleeve or case; if maximum speed matters most, accept some cosmetic wear and add at least an LCD protector.