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 $189.91   8.158%   $205.4029   $205.40   $205.40   $205.40  
  Purchase #2 $14.62   9.264%   $15.9744   $15.97   $15.95   $16.00  
  Purchase #3 $5.45   3.804%   $5.6573   $5.66   $5.65   $5.70  
  Purchase #4 $189.28   7.597%   $203.6596   $203.66   $203.65   $203.70  
  Purchase #5 $197.34   10.806%   $218.6646   $218.66   $218.65   $218.70  
  Purchase #6 $41.76   8.476%   $45.2996   $45.30   $45.30   $45.30  
  Purchase #7 $100.84   12.152%   $113.0941   $113.09   $113.10   $113.10  
  7 purchase totals = $739.20       $807.7525   $807.74   $807.70   $807.90