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 $16.67   9.124%   $18.191   $18.19   $18.20   $18.20  
  Purchase #2 $222.31   4.321%   $231.916   $231.92   $231.90   $231.90  
  Purchase #3 $191.54   5.167%   $201.4369   $201.44   $201.45   $201.40  
  Purchase #4 $10.30   8.248%   $11.1495   $11.15   $11.15   $11.10  
  Purchase #5 $27.48   4.777%   $28.7927   $28.79   $28.80   $28.80  
  Purchase #6 $96.68   3.808%   $100.3616   $100.36   $100.35   $100.40  
  Purchase #7 $47.99   5.701%   $50.7259   $50.73   $50.75   $50.70  
  7 purchase totals = $612.97       $642.5736   $642.58   $642.60   $642.50