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 $99.79   9.119%   $108.8899   $108.89   $108.90   $108.90  
  Purchase #2 $123.31   4.126%   $128.3978   $128.40   $128.40   $128.40  
  Purchase #3 $21.93   12.336%   $24.6353   $24.64   $24.65   $24.60  
  Purchase #4 $48.84   8.281%   $52.8844   $52.88   $52.90   $52.90  
  Purchase #5 $50.61   10.841%   $56.0966   $56.10   $56.10   $56.10  
  Purchase #6 $42.61   11.807%   $47.641   $47.64   $47.65   $47.60  
  Purchase #7 $209.62   7.609%   $225.57   $225.57   $225.55   $225.60  
  7 purchase totals = $596.71       $644.1150   $644.12   $644.15   $644.10