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 $126.64   7.481%   $136.1139   $136.11   $136.10   $136.10  
  Purchase #2 $234.76   9.871%   $257.9332   $257.93   $257.95   $257.90  
  Purchase #3 $114.47   8.134%   $123.781   $123.78   $123.80   $123.80  
  Purchase #4 $149.99   7.212%   $160.8073   $160.81   $160.80   $160.80  
  Purchase #5 $91.35   5.386%   $96.2701   $96.27   $96.25   $96.30  
  Purchase #6 $132.44   7.544%   $142.4313   $142.43   $142.45   $142.40  
  Purchase #7 $37.66   3.551%   $38.9973   $39.00   $39.00   $39.00  
  7 purchase totals = $887.31       $956.3341   $956.33   $956.35   $956.30