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 $128.52   3.814%   $133.4218   $133.42   $133.40   $133.40  
  Purchase #2 $51.37   12.469%   $57.7753   $57.78   $57.80   $57.80  
  Purchase #3 $224.90   5.646%   $237.5979   $237.60   $237.60   $237.60  
  Purchase #4 $176.58   11.816%   $197.4447   $197.44   $197.45   $197.40  
  Purchase #5 $30.33   9.288%   $33.1471   $33.15   $33.15   $33.10  
  Purchase #6 $16.99   9.381%   $18.5838   $18.58   $18.60   $18.60  
  Purchase #7 $110.82   8.230%   $119.9405   $119.94   $119.95   $119.90  
  7 purchase totals = $739.51       $797.9111   $797.91   $797.95   $797.80