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 $23.87   6.181%   $25.3454   $25.35   $25.35   $25.30  
  Purchase #2 $123.61   10.802%   $136.9624   $136.96   $136.95   $137.00  
  Purchase #3 $51.65   11.641%   $57.6626   $57.66   $57.65   $57.70  
  Purchase #4 $167.87   12.246%   $188.4274   $188.43   $188.45   $188.40  
  Purchase #5 $169.25   12.370%   $190.1862   $190.19   $190.20   $190.20  
  Purchase #6 $69.54   12.231%   $78.0454   $78.05   $78.05   $78.00  
  Purchase #7 $153.34   5.108%   $161.1726   $161.17   $161.15   $161.20  
  7 purchase totals = $759.13       $837.8020   $837.81   $837.80   $837.80