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 $20.30   5.820%   $21.4815   $21.48   $21.50   $21.50  
  Purchase #2 $23.98   12.547%   $26.9888   $26.99   $27.00   $27.00  
  Purchase #3 $81.33   10.506%   $89.8745   $89.87   $89.85   $89.90  
  Purchase #4 $149.31   11.362%   $166.2746   $166.27   $166.25   $166.30  
  Purchase #5 $63.37   13.259%   $71.7722   $71.77   $71.75   $71.80  
  Purchase #6 $32.48   5.799%   $34.3635   $34.36   $34.35   $34.40  
  Purchase #7 $214.99   4.486%   $224.6345   $224.63   $224.65   $224.60  
  7 purchase totals = $585.76       $635.3896   $635.37   $635.35   $635.50