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 $234.71   12.779%   $264.7036   $264.70   $264.70   $264.70  
  Purchase #2 $159.59   11.260%   $177.5598   $177.56   $177.55   $177.60  
  Purchase #3 $58.72   12.262%   $65.9202   $65.92   $65.90   $65.90  
  Purchase #4 $51.55   7.523%   $55.4281   $55.43   $55.45   $55.40  
  Purchase #5 $191.59   9.298%   $209.404   $209.40   $209.40   $209.40  
  Purchase #6 $83.57   10.727%   $92.5346   $92.53   $92.55   $92.50  
  Purchase #7 $244.40   11.473%   $272.44   $272.44   $272.45   $272.40  
  7 purchase totals = $1,024.13       $1,137.9903   $1,137.98   $1,138.00   $1,137.90