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 $29.79   8.514%   $32.3263   $32.33   $32.35   $32.30  
  Purchase #2 $126.37   6.201%   $134.2062   $134.21   $134.20   $134.20  
  Purchase #3 $99.41   9.551%   $108.9046   $108.90   $108.90   $108.90  
  Purchase #4 $240.44   13.860%   $273.765   $273.77   $273.75   $273.80  
  Purchase #5 $202.41   9.833%   $222.313   $222.31   $222.30   $222.30  
  Purchase #6 $202.34   4.429%   $211.3016   $211.30   $211.30   $211.30  
  Purchase #7 $15.98   6.473%   $17.0144   $17.01   $17.00   $17.00  
  7 purchase totals = $916.74       $999.8311   $999.83   $999.80   $999.80