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 $9.89   12.811%   $11.157   $11.16   $11.15   $11.20  
  Purchase #2 $102.65   4.125%   $106.8843   $106.88   $106.90   $106.90  
  Purchase #3 $238.88   4.612%   $249.8971   $249.90   $249.90   $249.90  
  Purchase #4 $112.56   4.276%   $117.3731   $117.37   $117.35   $117.40  
  Purchase #5 $142.81   11.127%   $158.7005   $158.70   $158.70   $158.70  
  Purchase #6 $11.31   8.578%   $12.2802   $12.28   $12.30   $12.30  
  Purchase #7 $31.49   4.242%   $32.8258   $32.83   $32.85   $32.80  
  7 purchase totals = $649.59       $689.1180   $689.12   $689.15   $689.20