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 $75.71   11.282%   $84.2516   $84.25   $84.25   $84.30  
  Purchase #2 $195.57   5.769%   $206.8524   $206.85   $206.85   $206.90  
  Purchase #3 $88.57   7.228%   $94.9718   $94.97   $94.95   $95.00  
  Purchase #4 $37.34   6.871%   $39.9056   $39.91   $39.90   $39.90  
  Purchase #5 $209.95   8.690%   $228.1947   $228.19   $228.20   $228.20  
  Purchase #6 $209.60   4.251%   $218.5101   $218.51   $218.50   $218.50  
  Purchase #7 $116.93   8.294%   $126.6282   $126.63   $126.65   $126.60  
  7 purchase totals = $933.67       $999.3144   $999.31   $999.30   $999.40