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 $20.73   4.319%   $21.6253   $21.63   $21.65   $21.60  
  Purchase #2 $63.36   10.440%   $69.9748   $69.97   $69.95   $70.00  
  Purchase #3 $208.85   9.229%   $228.1248   $228.12   $228.10   $228.10  
  Purchase #4 $46.80   7.718%   $50.412   $50.41   $50.40   $50.40  
  Purchase #5 $55.48   6.654%   $59.1716   $59.17   $59.15   $59.20  
  Purchase #6 $6.54   8.605%   $7.1028   $7.10   $7.10   $7.10  
  Purchase #7 $91.84   6.609%   $97.9097   $97.91   $97.90   $97.90  
  7 purchase totals = $493.60       $534.3210   $534.31   $534.25   $534.30