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 $47.93   6.531%   $51.0603   $51.06   $51.05   $51.10  
  Purchase #2 $246.29   13.503%   $279.5465   $279.55   $279.55   $279.50  
  Purchase #3 $209.98   7.577%   $225.8902   $225.89   $225.90   $225.90  
  Purchase #4 $77.30   7.392%   $83.014   $83.01   $83.00   $83.00  
  Purchase #5 $32.59   6.788%   $34.8022   $34.80   $34.80   $34.80  
  Purchase #6 $53.49   9.179%   $58.3998   $58.40   $58.40   $58.40  
  Purchase #7 $248.77   8.331%   $269.495   $269.50   $269.50   $269.50  
  7 purchase totals = $916.35       $1,002.2080   $1,002.21   $1,002.20   $1,002.20