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 $249.49   3.764%   $258.8808   $258.88   $258.90   $258.90  
  Purchase #2 $61.27   5.393%   $64.5743   $64.57   $64.55   $64.60  
  Purchase #3 $77.37   9.195%   $84.4842   $84.48   $84.50   $84.50  
  Purchase #4 $61.76   8.356%   $66.9207   $66.92   $66.90   $66.90  
  Purchase #5 $208.37   13.334%   $236.1541   $236.15   $236.15   $236.20  
  Purchase #6 $200.79   3.196%   $207.2072   $207.21   $207.20   $207.20  
  Purchase #7 $118.75   8.561%   $128.9162   $128.92   $128.90   $128.90  
  7 purchase totals = $977.80       $1,047.1375   $1,047.13   $1,047.10   $1,047.20