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 $74.91   12.139%   $84.0033   $84.00   $84.00   $84.00  
  Purchase #2 $112.78   4.325%   $117.6577   $117.66   $117.65   $117.70  
  Purchase #3 $154.56   11.302%   $172.0284   $172.03   $172.05   $172.00  
  Purchase #4 $93.34   12.198%   $104.7256   $104.73   $104.75   $104.70  
  Purchase #5 $211.93   10.355%   $233.8754   $233.88   $233.90   $233.90  
  Purchase #6 $231.61   9.756%   $254.2059   $254.21   $254.20   $254.20  
  Purchase #7 $22.60   9.333%   $24.7093   $24.71   $24.70   $24.70  
  7 purchase totals = $901.73       $991.2056   $991.22   $991.25   $991.20