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 $238.95   11.536%   $266.5153   $266.52   $266.50   $266.50  
  Purchase #2 $131.62   5.512%   $138.8749   $138.87   $138.85   $138.90  
  Purchase #3 $231.69   11.121%   $257.4562   $257.46   $257.45   $257.50  
  Purchase #4 $147.32   11.475%   $164.225   $164.23   $164.25   $164.20  
  Purchase #5 $6.63   3.787%   $6.8811   $6.88   $6.90   $6.90  
  Purchase #6 $86.56   10.684%   $95.8081   $95.81   $95.80   $95.80  
  Purchase #7 $91.32   4.764%   $95.6705   $95.67   $95.65   $95.70  
  7 purchase totals = $934.09       $1,025.4311   $1,025.44   $1,025.40   $1,025.50