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 $226.50   9.110%   $247.1342   $247.13   $247.15   $247.10  
  Purchase #2 $171.65   5.196%   $180.5689   $180.57   $180.55   $180.60  
  Purchase #3 $249.31   8.274%   $269.9379   $269.94   $269.95   $269.90  
  Purchase #4 $70.89   13.216%   $80.2588   $80.26   $80.25   $80.30  
  Purchase #5 $192.57   6.277%   $204.6576   $204.66   $204.65   $204.70  
  Purchase #6 $224.90   10.777%   $249.1375   $249.14   $249.15   $249.10  
  Purchase #7 $42.28   11.113%   $46.9786   $46.98   $47.00   $47.00  
  7 purchase totals = $1,178.10       $1,278.6735   $1,278.68   $1,278.70   $1,278.70