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 $28.78   4.645%   $30.1168   $30.12   $30.10   $30.10  
  Purchase #2 $76.37   6.350%   $81.2195   $81.22   $81.20   $81.20  
  Purchase #3 $127.78   11.241%   $142.1437   $142.14   $142.15   $142.10  
  Purchase #4 $27.31   4.224%   $28.4636   $28.46   $28.45   $28.50  
  Purchase #5 $157.80   8.410%   $171.071   $171.07   $171.05   $171.10  
  Purchase #6 $31.91   5.832%   $33.771   $33.77   $33.75   $33.80  
  Purchase #7 $136.30   13.762%   $155.0576   $155.06   $155.05   $155.10  
  7 purchase totals = $586.25       $641.8432   $641.84   $641.75   $641.90