Most reputable companies will honour a written offer but technically till the point there is a written contract between the parties nothing is certain so if you're the glass half empty type, I'd suggest you keep some irons in the fire. Generally speaking there is nothing to lose by doing that anyway.Generally offers tend to be conditional on things like references etc. anyway and you could also be dismissed the day you join - all these events are low probability events but obviously possible. In general if the offer is from a good company, a written offer should give you a reasonable feeling of certainty.
