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 $36.44   7.420%   $39.1438   $39.14   $39.15   $39.10  
  Purchase #2 $115.86   11.409%   $129.0785   $129.08   $129.10   $129.10  
  Purchase #3 $124.88   12.147%   $140.0492   $140.05   $140.05   $140.00  
  Purchase #4 $73.35   3.770%   $76.1153   $76.12   $76.10   $76.10  
  Purchase #5 $115.74   8.758%   $125.8765   $125.88   $125.90   $125.90  
  Purchase #6 $48.90   6.506%   $52.0814   $52.08   $52.10   $52.10  
  Purchase #7 $234.39   10.200%   $258.2978   $258.30   $258.30   $258.30  
  7 purchase totals = $749.56       $820.6425   $820.65   $820.70   $820.60