Jeez - I have the creeps now, just thinking of all the dodgy places I rented over 15 years in Dublin. I'm NEVER gonna rent again!
If you have ever stayed in a Hotel I can guarantee you the mattress in most of they could tell you a few stories...Horror stories!!
The first thing I do on checking in is removing the bedspread and put it to one side,I remember seeing a programme where they ran a UV light over one of them,it was covered in ahem bodily fluids not discernible to the eye...
I would not be replacing mattresses after each tenancy.