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 $18.85   13.417%   $21.3791   $21.38   $21.40   $21.40  
  Purchase #2 $15.41   4.291%   $16.0712   $16.07   $16.05   $16.10  
  Purchase #3 $74.86   8.748%   $81.4088   $81.41   $81.40   $81.40  
  Purchase #4 $42.48   7.363%   $45.6078   $45.61   $45.60   $45.60  
  Purchase #5 $141.55   8.184%   $153.1345   $153.13   $153.15   $153.10  
  Purchase #6 $128.84   7.411%   $138.3883   $138.39   $138.40   $138.40  
  Purchase #7 $183.62   12.850%   $207.2152   $207.22   $207.20   $207.20  
  7 purchase totals = $605.61       $663.2049   $663.21   $663.20   $663.20