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 $126.31   9.704%   $138.5671   $138.57   $138.55   $138.60  
  Purchase #2 $8.60   7.592%   $9.2529   $9.25   $9.25   $9.30  
  Purchase #3 $203.46   13.826%   $231.5904   $231.59   $231.60   $231.60  
  Purchase #4 $14.82   12.312%   $16.6446   $16.64   $16.65   $16.60  
  Purchase #5 $86.71   3.847%   $90.0457   $90.05   $90.05   $90.00  
  Purchase #6 $171.28   7.173%   $183.5659   $183.57   $183.55   $183.60  
  Purchase #7 $208.62   8.366%   $226.0731   $226.07   $226.05   $226.10  
  7 purchase totals = $819.80       $895.7397   $895.74   $895.70   $895.80