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 $16.82   7.668%   $18.1098   $18.11   $18.10   $18.10  
  Purchase #2 $34.58   13.167%   $39.1331   $39.13   $39.15   $39.10  
  Purchase #3 $138.85   11.468%   $154.7733   $154.77   $154.75   $154.80  
  Purchase #4 $193.72   11.664%   $216.3155   $216.32   $216.30   $216.30  
  Purchase #5 $27.74   9.259%   $30.3084   $30.31   $30.30   $30.30  
  Purchase #6 $58.39   4.308%   $60.9054   $60.91   $60.90   $60.90  
  Purchase #7 $8.90   12.405%   $10.004   $10.00   $10.00   $10.00  
  7 purchase totals = $479.00       $529.5495   $529.55   $529.50   $529.50