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 $29.53   4.268%   $30.7903   $30.79   $30.80   $30.80  
  Purchase #2 $33.28   12.696%   $37.5052   $37.51   $37.50   $37.50  
  Purchase #3 $209.93   12.861%   $236.9291   $236.93   $236.95   $236.90  
  Purchase #4 $227.97   3.531%   $236.0196   $236.02   $236.00   $236.00  
  Purchase #5 $155.44   5.353%   $163.7607   $163.76   $163.75   $163.80  
  Purchase #6 $11.27   4.583%   $11.7865   $11.79   $11.80   $11.80  
  Purchase #7 $20.42   11.374%   $22.7426   $22.74   $22.75   $22.70  
  7 purchase totals = $687.84       $739.5340   $739.54   $739.55   $739.50