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 $12.30   9.741%   $13.4981   $13.50   $13.50   $13.50  
  Purchase #2 $29.45   8.417%   $31.9288   $31.93   $31.95   $31.90  
  Purchase #3 $177.71   9.545%   $194.6724   $194.67   $194.65   $194.70  
  Purchase #4 $15.92   9.709%   $17.4657   $17.47   $17.45   $17.50  
  Purchase #5 $25.68   12.329%   $28.8461   $28.85   $28.85   $28.80  
  Purchase #6 $96.51   12.235%   $108.318   $108.32   $108.30   $108.30  
  Purchase #7 $123.81   7.618%   $133.2418   $133.24   $133.25   $133.20  
  7 purchase totals = $481.38       $527.9709   $527.98   $527.95   $527.90