should I cut the tops off sedums?

If you mean Sedum spectabile, the old shoots of this are usually cut back in early spring.

The ‘Chelsea chop’ was originally a nurseryman’s term for cutting back herbaceous plants around Chelsea show time to flower later and in more compact shape.

The term has been extended to use in the garden, cutting back plants , such as lupins and delphiniums that have flowered to get some later flowers. And clipping plants that have not flowered has been practiced to push flowering back into early autumn and leave more compct growth.