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 $54.74   6.503%   $58.2997   $58.30   $58.30   $58.30  
  Purchase #2 $16.86   6.778%   $18.0028   $18.00   $18.00   $18.00  
  Purchase #3 $54.47   11.153%   $60.545   $60.55   $60.55   $60.50  
  Purchase #4 $224.92   12.315%   $252.6189   $252.62   $252.60   $252.60  
  Purchase #5 $23.87   13.115%   $27.0006   $27.00   $27.00   $27.00  
  Purchase #6 $192.84   5.467%   $203.3826   $203.38   $203.40   $203.40  
  Purchase #7 $186.95   5.331%   $196.9163   $196.92   $196.90   $196.90  
  7 purchase totals = $754.65       $816.7659   $816.77   $816.75   $816.70