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 $100.70   8.225%   $108.9826   $108.98   $109.00   $109.00  
  Purchase #2 $34.78   10.797%   $38.5352   $38.54   $38.55   $38.50  
  Purchase #3 $94.75   10.426%   $104.6286   $104.63   $104.65   $104.60  
  Purchase #4 $21.43   3.117%   $22.098   $22.10   $22.10   $22.10  
  Purchase #5 $56.58   10.490%   $62.5152   $62.52   $62.50   $62.50  
  Purchase #6 $24.75   6.137%   $26.2689   $26.27   $26.25   $26.30  
  Purchase #7 $226.49   4.133%   $235.8508   $235.85   $235.85   $235.90  
  7 purchase totals = $559.48       $598.8793   $598.89   $598.90   $598.90