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 $131.63   4.366%   $137.377   $137.38   $137.40   $137.40  
  Purchase #2 $54.92   13.207%   $62.1733   $62.17   $62.15   $62.20  
  Purchase #3 $194.36   7.186%   $208.3267   $208.33   $208.35   $208.30  
  Purchase #4 $119.31   9.500%   $130.6445   $130.64   $130.65   $130.60  
  Purchase #5 $20.88   13.794%   $23.7602   $23.76   $23.75   $23.80  
  Purchase #6 $7.36   4.394%   $7.6834   $7.68   $7.70   $7.70  
  Purchase #7 $60.51   13.862%   $68.8979   $68.90   $68.90   $68.90  
  7 purchase totals = $588.97       $638.8630   $638.86   $638.90   $638.90