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 $57.51   13.268%   $65.1404   $65.14   $65.15   $65.10  
  Purchase #2 $48.25   12.518%   $54.2899   $54.29   $54.30   $54.30  
  Purchase #3 $63.25   5.607%   $66.7964   $66.80   $66.80   $66.80  
  Purchase #4 $14.39   13.653%   $16.3547   $16.35   $16.35   $16.40  
  Purchase #5 $235.63   5.762%   $249.207   $249.21   $249.20   $249.20  
  Purchase #6 $152.65   8.599%   $165.7764   $165.78   $165.80   $165.80  
  Purchase #7 $72.58   3.602%   $75.1943   $75.19   $75.20   $75.20  
  7 purchase totals = $644.26       $692.7591   $692.76   $692.80   $692.80