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 $19.70   5.425%   $20.7687   $20.77   $20.75   $20.80  
  Purchase #2 $43.77   8.658%   $47.5596   $47.56   $47.55   $47.60  
  Purchase #3 $30.75   4.541%   $32.1464   $32.15   $32.15   $32.10  
  Purchase #4 $12.25   10.319%   $13.5141   $13.51   $13.50   $13.50  
  Purchase #5 $68.62   10.282%   $75.6755   $75.68   $75.70   $75.70  
  Purchase #6 $229.98   8.266%   $248.9901   $248.99   $249.00   $249.00  
  Purchase #7 $80.26   11.235%   $89.2772   $89.28   $89.30   $89.30  
  7 purchase totals = $485.33       $527.9316   $527.94   $527.95   $528.00