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 $34.67   12.836%   $39.1202   $39.12   $39.10   $39.10  
  Purchase #2 $234.96   5.403%   $247.6549   $247.65   $247.65   $247.70  
  Purchase #3 $205.76   13.680%   $233.908   $233.91   $233.90   $233.90  
  Purchase #4 $67.41   9.436%   $73.7708   $73.77   $73.75   $73.80  
  Purchase #5 $185.56   11.323%   $206.571   $206.57   $206.55   $206.60  
  Purchase #6 $151.62   10.781%   $167.9662   $167.97   $167.95   $168.00  
  Purchase #7 $243.83   12.268%   $273.7431   $273.74   $273.75   $273.70  
  7 purchase totals = $1,123.81       $1,242.7342   $1,242.73   $1,242.65   $1,242.80