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 $121.82   10.566%   $134.6915   $134.69   $134.70   $134.70  
  Purchase #2 $182.57   7.476%   $196.2189   $196.22   $196.20   $196.20  
  Purchase #3 $153.59   8.488%   $166.6267   $166.63   $166.65   $166.60  
  Purchase #4 $45.44   12.164%   $50.9673   $50.97   $50.95   $51.00  
  Purchase #5 $222.89   10.829%   $247.0268   $247.03   $247.05   $247.00  
  Purchase #6 $13.79   5.663%   $14.5709   $14.57   $14.55   $14.60  
  Purchase #7 $158.78   6.862%   $169.6755   $169.68   $169.70   $169.70  
  7 purchase totals = $898.88       $979.7776   $979.79   $979.80   $979.80