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 $224.31   13.470%   $254.5246   $254.52   $254.50   $254.50  
  Purchase #2 $78.34   4.216%   $81.6428   $81.64   $81.65   $81.60  
  Purchase #3 $49.32   7.873%   $53.203   $53.20   $53.20   $53.20  
  Purchase #4 $59.26   9.287%   $64.7635   $64.76   $64.75   $64.80  
  Purchase #5 $14.91   3.528%   $15.436   $15.44   $15.45   $15.40  
  Purchase #6 $204.66   11.712%   $228.6298   $228.63   $228.65   $228.60  
  Purchase #7 $103.26   9.483%   $113.0521   $113.05   $113.05   $113.10  
  7 purchase totals = $734.06       $811.2518   $811.24   $811.25   $811.20