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 $109.73   13.848%   $124.9254   $124.93   $124.95   $124.90  
  Purchase #2 $84.70   7.551%   $91.0957   $91.10   $91.10   $91.10  
  Purchase #3 $168.82   9.747%   $185.2749   $185.27   $185.25   $185.30  
  Purchase #4 $41.46   5.521%   $43.749   $43.75   $43.75   $43.70  
  Purchase #5 $215.32   12.535%   $242.3104   $242.31   $242.30   $242.30  
  Purchase #6 $250.30   5.278%   $263.5108   $263.51   $263.50   $263.50  
  Purchase #7 $39.48   11.541%   $44.0364   $44.04   $44.05   $44.00  
  7 purchase totals = $909.81       $994.9026   $994.91   $994.90   $994.80