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 $15.61   3.835%   $16.2086   $16.21   $16.20   $16.20  
  Purchase #2 $203.86   13.727%   $231.8439   $231.84   $231.85   $231.80  
  Purchase #3 $67.85   8.330%   $73.5019   $73.50   $73.50   $73.50  
  Purchase #4 $146.79   6.462%   $156.2756   $156.28   $156.30   $156.30  
  Purchase #5 $142.78   10.574%   $157.8776   $157.88   $157.90   $157.90  
  Purchase #6 $129.27   9.525%   $141.583   $141.58   $141.60   $141.60  
  Purchase #7 $210.90   6.127%   $223.8218   $223.82   $223.80   $223.80  
  7 purchase totals = $917.06       $1,001.1124   $1,001.11   $1,001.15   $1,001.10