Starting out?

My wife and I are planning a RTW trip covering S.America and Asia including Nepal and other cold areas. We will be staying in hostels throughout the trip. Should we take sleeping bags or just silk sheets or both? I am not

I would definitely take a sleeping bag for Nepal. You can hire one if you want or rely on blankets in tea houses, which aren't always particularly clean, but you're really better off with your own bag. You could always flog it after you've finished with cold places. I'd also take a lightweight liner to keep the bag reasonably clean.

I would say yes,A comfy,clean sleeping bag on A long trip is nearly always worth the space.Even hot countries can have cool areas/nights.

Yes definitely take one.

yes

Definitely take one to Nepal, with a liner (comfort rating of -10 should keep you both warm) . Then ship it back to uk, if you are not doing anymore trekking, if you plan to trek in other places it will be worth keeping it with you. Don't ship from Nepal it may well never get home.
If you are just doing normal (hostel/hotel) RTW travelling you won't use you bag, as nearly all places ahve clean bed linen (if they don't change hostel). It will just become another thing to carry.

Definitely take good sleeping bags - the warmer the better. I did a RTW trip 2000-2001 and took a synthetic bag and fleece/jacket. I did the Everest and Annapurnas and it was freezing at night. Once the sun goes behind those mountains, kiss goodbye to warmth and comfort. I wish I would have taken a DOWN BAG and a DOWN Jacket. In the lodges during your trek they have a fire but only enough fuel for 2 hours heat at the most. When that goes out its bed (at about 8pm). And remember the higher up you go the colder it gets.

I did many other treks. I hit New Zealand as winter was approaching - again my synthetic kit was not sufficient. When I knew I would not be using my trekking gear for a while I posted it on to the next country and picked it up. The Post Restante system is fantastic.

Have a memorable time. The Himalayas are without doubt a spectaular place. The cold nights and the hardship just add extra specialness to the experience. It was truly worth every moment of the cold!

PS. I have now invested in the best Down sleeping bag and jacket but could not afford them at the time!