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 $170.48   3.108%   $175.7785   $175.78   $175.80   $175.80  
  Purchase #2 $27.51   4.114%   $28.6418   $28.64   $28.65   $28.60  
  Purchase #3 $31.91   12.116%   $35.7762   $35.78   $35.80   $35.80  
  Purchase #4 $29.45   4.487%   $30.7714   $30.77   $30.75   $30.80  
  Purchase #5 $124.62   9.857%   $136.9038   $136.90   $136.90   $136.90  
  Purchase #6 $63.78   9.241%   $69.6739   $69.67   $69.65   $69.70  
  Purchase #7 $156.40   5.740%   $165.3774   $165.38   $165.40   $165.40  
  7 purchase totals = $604.15       $642.9230   $642.92   $642.95   $643.00