Dani is right. However, the drop in RI is pretty slim, so it really depends on what type of ops you're running.
Mediums are virtually immune to RI drops, even during a D-check.
Larges can lose a few points, but you'll barely feel it.
VLs however can lose quite a few points during a D. And if you're flying cargo... I once had a route on classic scheduling that dropped to 73. Due to how RI attracts cargo, it took me 3 months after the check to get back to previous LFs.
So yes, for VLs and especially cargo, loss of RI definitely is an issue. For the rest... Meh...