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 $79.75   12.432%   $89.6645   $89.66   $89.65   $89.70  
  Purchase #2 $139.52   6.186%   $148.1507   $148.15   $148.15   $148.20  
  Purchase #3 $11.32   7.124%   $12.1264   $12.13   $12.15   $12.10  
  Purchase #4 $36.44   4.668%   $38.141   $38.14   $38.15   $38.10  
  Purchase #5 $115.31   12.615%   $129.8564   $129.86   $129.85   $129.90  
  Purchase #6 $138.46   6.763%   $147.824   $147.82   $147.80   $147.80  
  Purchase #7 $37.63   5.331%   $39.6361   $39.64   $39.65   $39.60  
  7 purchase totals = $558.43       $605.3991   $605.40   $605.40   $605.40