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 $20.26   6.383%   $21.5532   $21.55   $21.55   $21.60  
  Purchase #2 $65.76   10.494%   $72.6609   $72.66   $72.65   $72.70  
  Purchase #3 $10.34   9.213%   $11.2926   $11.29   $11.30   $11.30  
  Purchase #4 $102.68   6.558%   $109.4138   $109.41   $109.40   $109.40  
  Purchase #5 $178.93   5.633%   $189.0091   $189.01   $189.00   $189.00  
  Purchase #6 $19.26   3.367%   $19.9085   $19.91   $19.90   $19.90  
  Purchase #7 $60.78   8.765%   $66.1074   $66.11   $66.10   $66.10  
  7 purchase totals = $458.01       $489.9455   $489.94   $489.90   $490.00