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 $139.51   7.447%   $149.8993   $149.90   $149.90   $149.90  
  Purchase #2 $119.48   8.596%   $129.7505   $129.75   $129.75   $129.80  
  Purchase #3 $63.53   4.470%   $66.3698   $66.37   $66.35   $66.40  
  Purchase #4 $12.52   4.743%   $13.1138   $13.11   $13.10   $13.10  
  Purchase #5 $34.37   5.825%   $36.3721   $36.37   $36.35   $36.40  
  Purchase #6 $192.77   9.148%   $210.4046   $210.40   $210.40   $210.40  
  Purchase #7 $31.33   10.808%   $34.7161   $34.72   $34.70   $34.70  
  7 purchase totals = $593.51       $640.6262   $640.62   $640.55   $640.70