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 $243.75   13.128%   $275.7495   $275.75   $275.75   $275.70  
  Purchase #2 $224.96   10.838%   $249.3412   $249.34   $249.35   $249.30  
  Purchase #3 $46.53   9.384%   $50.8964   $50.90   $50.90   $50.90  
  Purchase #4 $56.27   4.540%   $58.8247   $58.82   $58.80   $58.80  
  Purchase #5 $117.77   3.570%   $121.9744   $121.97   $121.95   $122.00  
  Purchase #6 $8.40   4.703%   $8.7951   $8.80   $8.80   $8.80  
  Purchase #7 $250.44   6.776%   $267.4098   $267.41   $267.40   $267.40  
  7 purchase totals = $948.12       $1,032.9911   $1,032.99   $1,032.95   $1,032.90