Guide for the Care and Use of Laboratory Animals:
page 24 –

"Rodents are often housed on wire flooring, which enhances sanitation of the cage by enabling urine and feces to pass through to a collection tray. However, some evidence suggests that solid-bottom caging, with bedding, is preferred by rodents (Fullerton and Gilliatt 1967; Grover-Johnson and Spencer 1981; Ortman and others1983). Solid-bottom caging, with bedding, is therefore recommended for rodents."