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 $51.81   11.453%   $57.7438   $57.74   $57.75   $57.70  
  Purchase #2 $53.83   11.173%   $59.8444   $59.84   $59.85   $59.80  
  Purchase #3 $80.62   4.838%   $84.5204   $84.52   $84.50   $84.50  
  Purchase #4 $212.74   3.603%   $220.405   $220.41   $220.40   $220.40  
  Purchase #5 $28.56   6.685%   $30.4692   $30.47   $30.45   $30.50  
  Purchase #6 $225.90   4.334%   $235.6905   $235.69   $235.70   $235.70  
  Purchase #7 $238.93   13.234%   $270.55   $270.55   $270.55   $270.60  
  7 purchase totals = $892.39       $959.2233   $959.22   $959.20   $959.20