Hey Stevie,
The MergeONNextSyncTo field simply asks Quickbooks to merge the records on the next sync. The merge is done by Quickbooks, not Method. Since leads exist in Method only, and not Quickbooks, you cannot merge leads. Do some testing before you start merging so you know exactly how Quickbooks will merge the data.
However, you can use the Import/Export tool and Excel to clean up the data and reimport it into Method. Here is how I would approach the issue:
1) Go to Customize -> Integration Tools -> Import / Export
2) Select Export a table to a file
3) From the drop down menu select Customer and Customer Lead
4) Select Comma Seperated Values, and click Export File.
At this point you use the exported CSV file in Excel to merge records as you need. When the data has been cleaned up, you can use the same Import / Export screen to performa modify import, which will update the customers you changed in Excel. Then, you can simply delete the leads that are no longer needed.
Sam