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 $11.81   12.468%   $13.2825   $13.28   $13.30   $13.30  
  Purchase #2 $232.74   10.619%   $257.4547   $257.45   $257.45   $257.50  
  Purchase #3 $210.27   11.620%   $234.7034   $234.70   $234.70   $234.70  
  Purchase #4 $250.26   4.504%   $261.5317   $261.53   $261.55   $261.50  
  Purchase #5 $167.56   6.831%   $179.006   $179.01   $179.00   $179.00  
  Purchase #6 $63.45   5.274%   $66.7964   $66.80   $66.80   $66.80  
  Purchase #7 $39.68   3.696%   $41.1466   $41.15   $41.15   $41.10  
  7 purchase totals = $975.77       $1,053.9213   $1,053.92   $1,053.95   $1,053.90