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 $110.35   11.594%   $123.144   $123.14   $123.15   $123.10  
  Purchase #2 $14.56   12.162%   $16.3308   $16.33   $16.35   $16.30  
  Purchase #3 $243.39   10.706%   $269.4473   $269.45   $269.45   $269.40  
  Purchase #4 $124.57   6.169%   $132.2547   $132.25   $132.25   $132.30  
  Purchase #5 $107.89   10.401%   $119.1116   $119.11   $119.10   $119.10  
  Purchase #6 $67.44   13.840%   $76.7737   $76.77   $76.75   $76.80  
  Purchase #7 $118.42   3.784%   $122.901   $122.90   $122.90   $122.90  
  7 purchase totals = $786.62       $859.9631   $859.95   $859.95   $859.90