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 $18.90   6.838%   $20.1924   $20.19   $20.20   $20.20  
  Purchase #2 $13.78   13.178%   $15.5959   $15.60   $15.60   $15.60  
  Purchase #3 $194.56   7.469%   $209.0917   $209.09   $209.10   $209.10  
  Purchase #4 $212.39   3.430%   $219.675   $219.68   $219.70   $219.70  
  Purchase #5 $131.74   12.271%   $147.9058   $147.91   $147.90   $147.90  
  Purchase #6 $28.87   13.374%   $32.7311   $32.73   $32.75   $32.70  
  Purchase #7 $47.41   9.612%   $51.967   $51.97   $51.95   $52.00  
  7 purchase totals = $647.65       $697.1589   $697.17   $697.20   $697.20