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 $18.63   10.330%   $20.5545   $20.55   $20.55   $20.60  
  Purchase #2 $119.80   13.722%   $136.239   $136.24   $136.25   $136.20  
  Purchase #3 $238.28   3.291%   $246.1218   $246.12   $246.10   $246.10  
  Purchase #4 $38.95   11.153%   $43.2941   $43.29   $43.30   $43.30  
  Purchase #5 $178.37   5.194%   $187.6345   $187.63   $187.65   $187.60  
  Purchase #6 $168.56   11.481%   $187.9124   $187.91   $187.90   $187.90  
  Purchase #7 $6.85   9.618%   $7.5088   $7.51   $7.50   $7.50  
  7 purchase totals = $769.44       $829.2651   $829.25   $829.25   $829.20