Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $117.61   10.533%   $129.9979   $130.00   $130.00   $130.00  
  Purchase #2 $147.85   13.570%   $167.9132   $167.91   $167.90   $167.90  
  Purchase #3 $5.44   7.724%   $5.8602   $5.86   $5.85   $5.90  
  Purchase #4 $172.38   10.492%   $190.4661   $190.47   $190.45   $190.50  
  Purchase #5 $12.94   3.500%   $13.3929   $13.39   $13.40   $13.40  
  Purchase #6 $103.32   11.307%   $115.0024   $115.00   $115.00   $115.00  
  Purchase #7 $20.62   5.223%   $21.697   $21.70   $21.70   $21.70  
  7 purchase totals = $580.16       $644.3297   $644.33   $644.30   $644.40