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 $20.76   13.697%   $23.6035   $23.60   $23.60   $23.60  
  Purchase #2 $228.92   10.288%   $252.4713   $252.47   $252.45   $252.50  
  Purchase #3 $42.45   10.440%   $46.8818   $46.88   $46.90   $46.90  
  Purchase #4 $240.52   12.592%   $270.8063   $270.81   $270.80   $270.80  
  Purchase #5 $26.92   3.307%   $27.8102   $27.81   $27.80   $27.80  
  Purchase #6 $199.96   6.802%   $213.5613   $213.56   $213.55   $213.60  
  Purchase #7 $14.28   7.319%   $15.3252   $15.33   $15.35   $15.30  
  7 purchase totals = $773.81       $850.4596   $850.46   $850.45   $850.50