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 $148.32   12.402%   $166.7146   $166.71   $166.70   $166.70  
  Purchase #2 $79.55   6.260%   $84.5298   $84.53   $84.55   $84.50  
  Purchase #3 $118.74   10.650%   $131.3858   $131.39   $131.40   $131.40  
  Purchase #4 $102.66   10.202%   $113.1334   $113.13   $113.15   $113.10  
  Purchase #5 $161.76   4.476%   $169.0004   $169.00   $169.00   $169.00  
  Purchase #6 $190.92   5.551%   $201.518   $201.52   $201.50   $201.50  
  Purchase #7 $97.42   9.838%   $107.0042   $107.00   $107.00   $107.00  
  7 purchase totals = $899.37       $973.2862   $973.28   $973.30   $973.20