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 $224.62   11.834%   $251.2015   $251.20   $251.20   $251.20  
  Purchase #2 $105.41   12.248%   $118.3206   $118.32   $118.30   $118.30  
  Purchase #3 $38.38   12.257%   $43.0842   $43.08   $43.10   $43.10  
  Purchase #4 $78.57   8.362%   $85.14   $85.14   $85.15   $85.10  
  Purchase #5 $188.90   13.149%   $213.7385   $213.74   $213.75   $213.70  
  Purchase #6 $206.73   5.247%   $217.5771   $217.58   $217.60   $217.60  
  Purchase #7 $150.28   6.487%   $160.0287   $160.03   $160.05   $160.00  
  7 purchase totals = $992.89       $1,089.0906   $1,089.09   $1,089.15   $1,089.00