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 $146.77   12.133%   $164.5776   $164.58   $164.60   $164.60  
  Purchase #2 $233.64   11.256%   $259.9385   $259.94   $259.95   $259.90  
  Purchase #3 $95.33   7.680%   $102.6513   $102.65   $102.65   $102.70  
  Purchase #4 $68.77   9.865%   $75.5542   $75.55   $75.55   $75.60  
  Purchase #5 $163.88   10.707%   $181.4266   $181.43   $181.45   $181.40  
  Purchase #6 $150.81   4.838%   $158.1062   $158.11   $158.10   $158.10  
  Purchase #7 $32.73   10.619%   $36.2056   $36.21   $36.20   $36.20  
  7 purchase totals = $891.93       $978.4600   $978.47   $978.50   $978.50