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 $9.48   9.414%   $10.3724   $10.37   $10.35   $10.40  
  Purchase #2 $5.29   11.265%   $5.8859   $5.89   $5.90   $5.90  
  Purchase #3 $22.30   12.791%   $25.1524   $25.15   $25.15   $25.20  
  Purchase #4 $10.41   10.224%   $11.4743   $11.47   $11.45   $11.50  
  Purchase #5 $12.60   11.556%   $14.0561   $14.06   $14.05   $14.10  
  Purchase #6 $109.43   12.122%   $122.6951   $122.70   $122.70   $122.70  
  Purchase #7 $126.91   3.121%   $130.8709   $130.87   $130.85   $130.90  
  7 purchase totals = $296.42       $320.5071   $320.51   $320.45   $320.70