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 $79.77   4.420%   $83.2958   $83.30   $83.30   $83.30  
  Purchase #2 $122.37   12.591%   $137.7776   $137.78   $137.80   $137.80  
  Purchase #3 $43.69   6.185%   $46.3922   $46.39   $46.40   $46.40  
  Purchase #4 $239.39   11.671%   $267.3292   $267.33   $267.35   $267.30  
  Purchase #5 $205.87   10.637%   $227.7684   $227.77   $227.75   $227.80  
  Purchase #6 $206.53   12.248%   $231.8258   $231.83   $231.85   $231.80  
  Purchase #7 $67.95   10.833%   $75.311   $75.31   $75.30   $75.30  
  7 purchase totals = $965.57       $1,069.7000   $1,069.71   $1,069.75   $1,069.70