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 $238.56   9.109%   $260.2904   $260.29   $260.30   $260.30  
  Purchase #2 $179.42   9.863%   $197.1162   $197.12   $197.10   $197.10  
  Purchase #3 $69.47   10.180%   $76.542   $76.54   $76.55   $76.50  
  Purchase #4 $62.80   11.187%   $69.8254   $69.83   $69.85   $69.80  
  Purchase #5 $51.82   10.638%   $57.3326   $57.33   $57.35   $57.30  
  Purchase #6 $131.70   7.497%   $141.5735   $141.57   $141.55   $141.60  
  Purchase #7 $70.63   7.377%   $75.8404   $75.84   $75.85   $75.80  
  7 purchase totals = $804.40       $878.5205   $878.52   $878.55   $878.40