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 $36.31   7.815%   $39.1476   $39.15   $39.15   $39.10  
  Purchase #2 $47.75   9.535%   $52.303   $52.30   $52.30   $52.30  
  Purchase #3 $22.66   5.550%   $23.9176   $23.92   $23.90   $23.90  
  Purchase #4 $128.31   9.204%   $140.1197   $140.12   $140.10   $140.10  
  Purchase #5 $106.57   12.322%   $119.7016   $119.70   $119.70   $119.70  
  Purchase #6 $234.25   4.794%   $245.4799   $245.48   $245.50   $245.50  
  Purchase #7 $91.89   8.618%   $99.8091   $99.81   $99.80   $99.80  
  7 purchase totals = $667.74       $720.4785   $720.48   $720.45   $720.40