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 $39.79   7.850%   $42.9135   $42.91   $42.90   $42.90  
  Purchase #2 $37.48   10.173%   $41.2928   $41.29   $41.30   $41.30  
  Purchase #3 $242.57   6.499%   $258.3346   $258.33   $258.35   $258.30  
  Purchase #4 $244.60   6.858%   $261.3747   $261.37   $261.35   $261.40  
  Purchase #5 $28.77   9.102%   $31.3886   $31.39   $31.40   $31.40  
  Purchase #6 $165.51   7.409%   $177.7726   $177.77   $177.75   $177.80  
  Purchase #7 $73.84   3.397%   $76.3483   $76.35   $76.35   $76.30  
  7 purchase totals = $832.56       $889.4251   $889.41   $889.40   $889.40