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 $10.51   3.779%   $10.9072   $10.91   $10.90   $10.90  
  Purchase #2 $123.46   8.426%   $133.8627   $133.86   $133.85   $133.90  
  Purchase #3 $102.33   5.663%   $108.1249   $108.12   $108.10   $108.10  
  Purchase #4 $122.50   6.795%   $130.8239   $130.82   $130.80   $130.80  
  Purchase #5 $108.64   8.452%   $117.8223   $117.82   $117.80   $117.80  
  Purchase #6 $197.77   7.258%   $212.1241   $212.12   $212.10   $212.10  
  Purchase #7 $108.60   13.163%   $122.895   $122.90   $122.90   $122.90  
  7 purchase totals = $773.81       $836.5601   $836.55   $836.45   $836.50