I'm a landlord & I would rather not view a property with a tenant in it. I would rather have the place cleaned up & painted etc first. If a tenant wanted to break a lease early & the place was tidy, then I might ask them.
But if a tenant is paying their rent & is not in breach of a lease, I would assume they are entitled to full & exclusive use of the property and not have to fit in with viewings as per the landlord or their agent.
Bambam, if you do agree to allow viewings, make sure you set up some very clear guidelines about viewings, notice etc and you should ask for a reduction in the rent as compensation.