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 $187.78   6.116%   $199.2646   $199.26   $199.25   $199.30  
  Purchase #2 $54.59   3.114%   $56.2899   $56.29   $56.30   $56.30  
  Purchase #3 $25.37   6.446%   $27.0054   $27.01   $27.00   $27.00  
  Purchase #4 $141.67   10.303%   $156.2663   $156.27   $156.25   $156.30  
  Purchase #5 $84.63   3.861%   $87.8976   $87.90   $87.90   $87.90  
  Purchase #6 $179.74   13.865%   $204.661   $204.66   $204.65   $204.70  
  Purchase #7 $94.89   10.606%   $104.954   $104.95   $104.95   $105.00  
  7 purchase totals = $768.67       $836.3388   $836.34   $836.30   $836.50