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 $19.90   4.535%   $20.8025   $20.80   $20.80   $20.80  
  Purchase #2 $184.40   11.724%   $206.0191   $206.02   $206.00   $206.00  
  Purchase #3 $15.43   12.416%   $17.3458   $17.35   $17.35   $17.30  
  Purchase #4 $116.33   8.420%   $126.125   $126.13   $126.15   $126.10  
  Purchase #5 $186.63   9.791%   $204.9029   $204.90   $204.90   $204.90  
  Purchase #6 $49.58   3.776%   $51.4521   $51.45   $51.45   $51.50  
  Purchase #7 $24.36   6.498%   $25.9429   $25.94   $25.95   $25.90  
  7 purchase totals = $596.63       $652.5903   $652.59   $652.60   $652.50