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 $49.79   12.385%   $55.9565   $55.96   $55.95   $56.00  
  Purchase #2 $62.92   6.149%   $66.789   $66.79   $66.80   $66.80  
  Purchase #3 $250.73   5.851%   $265.4002   $265.40   $265.40   $265.40  
  Purchase #4 $21.74   4.345%   $22.6846   $22.68   $22.70   $22.70  
  Purchase #5 $158.74   11.658%   $177.2459   $177.25   $177.25   $177.20  
  Purchase #6 $124.62   9.807%   $136.8415   $136.84   $136.85   $136.80  
  Purchase #7 $50.68   3.835%   $52.6236   $52.62   $52.60   $52.60  
  7 purchase totals = $719.22       $777.5413   $777.54   $777.55   $777.50