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 $19.91   5.805%   $21.0658   $21.07   $21.05   $21.10  
  Purchase #2 $23.31   7.873%   $25.1452   $25.15   $25.15   $25.10  
  Purchase #3 $65.80   4.573%   $68.809   $68.81   $68.80   $68.80  
  Purchase #4 $213.27   6.520%   $227.1752   $227.18   $227.20   $227.20  
  Purchase #5 $26.55   5.599%   $28.0365   $28.04   $28.05   $28.00  
  Purchase #6 $72.26   12.177%   $81.0591   $81.06   $81.05   $81.10  
  Purchase #7 $141.53   11.165%   $157.3318   $157.33   $157.35   $157.30  
  7 purchase totals = $562.63       $608.6226   $608.64   $608.65   $608.60