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 $44.89   12.528%   $50.5138   $50.51   $50.50   $50.50  
  Purchase #2 $171.28   12.481%   $192.6575   $192.66   $192.65   $192.70  
  Purchase #3 $13.71   4.106%   $14.2729   $14.27   $14.25   $14.30  
  Purchase #4 $197.40   3.232%   $203.78   $203.78   $203.80   $203.80  
  Purchase #5 $65.70   12.279%   $73.7673   $73.77   $73.75   $73.80  
  Purchase #6 $85.29   9.433%   $93.3354   $93.34   $93.35   $93.30  
  Purchase #7 $62.36   6.673%   $66.5213   $66.52   $66.50   $66.50  
  7 purchase totals = $640.63       $694.8482   $694.85   $694.80   $694.90