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 $239.46   8.573%   $259.9889   $259.99   $260.00   $260.00  
  Purchase #2 $136.71   9.862%   $150.1923   $150.19   $150.20   $150.20  
  Purchase #3 $135.77   4.343%   $141.6665   $141.67   $141.65   $141.70  
  Purchase #4 $166.45   4.388%   $173.7538   $173.75   $173.75   $173.80  
  Purchase #5 $24.84   7.612%   $26.7308   $26.73   $26.75   $26.70  
  Purchase #6 $14.28   11.483%   $15.9198   $15.92   $15.90   $15.90  
  Purchase #7 $226.62   9.230%   $247.537   $247.54   $247.55   $247.50  
  7 purchase totals = $944.13       $1,015.7891   $1,015.79   $1,015.80   $1,015.80