I'm using Dell Inspiron 14R and it can be upgraded(recommended by Dell) to Windows 8. Recently, Dell released latest Bios! The latest bios is for Windows 8 or for Windows 7?
It shouldn't matter, updates to a BIOS very rarely make a difference to the operating system.
For what it's worth, Dell publish information about Windows 8 support for their laptops here. According to the page, all the Inspiron 14R laptops support Windows 8.
If you know your specific model number you can review the notes for each model, including any particular issues that may arise after the upgrade. Some models have issues, and there are workarounds available. For example the one of the Inspiron 14R models has this note:
Bios is rather independent of OS (Or your UEFI bootloader, which more modern systems use) - it prepares the system for the OS to boot then stays out of the way. Unless there's a specific mention of something useful for windows 8 (which is unlikely), your old bios should be fine.
You can go either way - just remember to make sure you do the bios/uefi update safely, if you choose to go that way
Bioses are not specific to operating systems, although sometimes they may fix bugs.
Read the BIOS release notes and see what it says. If it mentions that this fixes problems with Windows 8, you may need to upgrade.
In your case this says
Because this is 2012 and not 1997, bios upgrades are rather painless and you should do it just to fix whatever minor problems they addrssed.