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 $243.93   12.220%   $273.7382   $273.74   $273.75   $273.70  
  Purchase #2 $17.90   6.644%   $19.0893   $19.09   $19.10   $19.10  
  Purchase #3 $27.39   8.541%   $29.7294   $29.73   $29.75   $29.70  
  Purchase #4 $140.63   12.551%   $158.2805   $158.28   $158.30   $158.30  
  Purchase #5 $106.62   12.513%   $119.9614   $119.96   $119.95   $120.00  
  Purchase #6 $194.78   9.329%   $212.951   $212.95   $212.95   $213.00  
  Purchase #7 $147.25   10.836%   $163.206   $163.21   $163.20   $163.20  
  7 purchase totals = $878.50       $976.9558   $976.96   $977.00   $977.00