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 $125.99   10.219%   $138.8649   $138.86   $138.85   $138.90  
  Purchase #2 $96.74   4.256%   $100.8573   $100.86   $100.85   $100.90  
  Purchase #3 $136.33   4.110%   $141.9332   $141.93   $141.95   $141.90  
  Purchase #4 $243.36   11.158%   $270.5141   $270.51   $270.50   $270.50  
  Purchase #5 $183.97   6.211%   $195.3964   $195.40   $195.40   $195.40  
  Purchase #6 $46.59   11.613%   $52.0005   $52.00   $52.00   $52.00  
  Purchase #7 $72.92   4.276%   $76.0381   $76.04   $76.05   $76.00  
  7 purchase totals = $905.90       $975.6045   $975.60   $975.60   $975.60