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 $126.53   6.142%   $134.3015   $134.30   $134.30   $134.30  
  Purchase #2 $22.51   10.563%   $24.8877   $24.89   $24.90   $24.90  
  Purchase #3 $58.46   13.130%   $66.1358   $66.14   $66.15   $66.10  
  Purchase #4 $236.49   9.620%   $259.2403   $259.24   $259.25   $259.20  
  Purchase #5 $153.54   11.772%   $171.6147   $171.61   $171.60   $171.60  
  Purchase #6 $165.35   9.621%   $181.2583   $181.26   $181.25   $181.30  
  Purchase #7 $72.71   9.782%   $79.8225   $79.82   $79.80   $79.80  
  7 purchase totals = $835.59       $917.2608   $917.26   $917.25   $917.20