Mint4win (and wubi) use the windows boot mechanism, and then once you select them from the windows boot menu it goes to grub, so I doubt that it'd affect the mbr. I've done more mint4win/wubi installs than I can count and they've never messed up the mbr. Actual dual bootin (with partitioning and everything) is a completely different story. Just installing LM would mess up the MBR (it would install grub to the mbr). Everything would work fine, but I've heard of problems with updates. I've also heard of problems with mint4win/wubi that have to do with updating windows (even to a new service pack). You have to do some work to get an install back after that. But mint4win is safe to use as far as I know (and from experience).
Dell XPS 15 l502x - Debian Testing 64-bit NetInst Xfce, SolydX 64-bit Debian Testing, SolydK 64-bit SolydXK Testing
Old Gateway Pentium 4 Desktop - Arch Linux 64-bit Xfce and SolydX 32-bit Sid