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 $91.30   8.555%   $99.1107   $99.11   $99.10   $99.10  
  Purchase #2 $21.36   3.238%   $22.0516   $22.05   $22.05   $22.10  
  Purchase #3 $175.38   8.259%   $189.8646   $189.86   $189.85   $189.90  
  Purchase #4 $142.75   4.702%   $149.4621   $149.46   $149.45   $149.50  
  Purchase #5 $147.77   5.590%   $156.0303   $156.03   $156.05   $156.00  
  Purchase #6 $144.58   8.692%   $157.1469   $157.15   $157.15   $157.10  
  Purchase #7 $157.37   3.625%   $163.0747   $163.07   $163.05   $163.10  
  7 purchase totals = $880.51       $936.7409   $936.73   $936.70   $936.80