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 $181.30   10.806%   $200.8913   $200.89   $200.90   $200.90  
  Purchase #2 $12.26   12.652%   $13.8111   $13.81   $13.80   $13.80  
  Purchase #3 $198.75   3.245%   $205.1994   $205.20   $205.20   $205.20  
  Purchase #4 $228.37   10.507%   $252.3648   $252.36   $252.35   $252.40  
  Purchase #5 $112.78   12.666%   $127.0647   $127.06   $127.05   $127.10  
  Purchase #6 $19.95   3.450%   $20.6383   $20.64   $20.65   $20.60  
  Purchase #7 $35.50   5.259%   $37.3669   $37.37   $37.35   $37.40  
  7 purchase totals = $788.91       $857.3365   $857.33   $857.30   $857.40