During the last turnover of my rental, I replaced ALL of the linoleum and a good portion of the carpet in the downstairs with porcelain tile for easier maintenance going forward.  Duh, I completely forgot to SEAL the grout. (Perhaps it was because the tile guy took so long to complete the job?! and he didn't mention it either. But I know better :( ) 
Now the first tenant that enjoyed the new tile is gone, and there are areas where the grout is so dark.  I am 100% sure that they never wet mopped any of the tile in the 8 months they were there. AND they cooked everything with oil - the major area of soil is the dining area. 
Is it worth it at this point to:
- Pay someone to clean the grout? 
- If so, can they really get it close to its original color? And can you recommend someone who can? 
- If so, is it worth it to then seal the grout at that point? 
Or is the grout just going to continue to get grimy because tenants won't keep it clean?
Or should I first try to treat the stained grout on my own with bleach?! Worth a shot?
Thanks, Paula