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 $16.75   7.804%   $18.0572   $18.06   $18.05   $18.10  
  Purchase #2 $28.96   8.501%   $31.4219   $31.42   $31.40   $31.40  
  Purchase #3 $201.35   5.699%   $212.8249   $212.82   $212.80   $212.80  
  Purchase #4 $199.94   3.357%   $206.652   $206.65   $206.65   $206.70  
  Purchase #5 $94.64   13.262%   $107.1912   $107.19   $107.20   $107.20  
  Purchase #6 $132.56   5.125%   $139.3537   $139.35   $139.35   $139.40  
  Purchase #7 $234.52   6.499%   $249.7615   $249.76   $249.75   $249.80  
  7 purchase totals = $908.72       $965.2624   $965.25   $965.20   $965.40