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 $27.68   8.678%   $30.0821   $30.08   $30.10   $30.10  
  Purchase #2 $25.85   13.780%   $29.4121   $29.41   $29.40   $29.40  
  Purchase #3 $48.79   12.306%   $54.7941   $54.79   $54.80   $54.80  
  Purchase #4 $26.37   3.817%   $27.3765   $27.38   $27.40   $27.40  
  Purchase #5 $76.61   4.803%   $80.2896   $80.29   $80.30   $80.30  
  Purchase #6 $109.96   8.843%   $119.6838   $119.68   $119.70   $119.70  
  Purchase #7 $125.31   7.354%   $134.5253   $134.53   $134.55   $134.50  
  7 purchase totals = $440.57       $476.1635   $476.16   $476.25   $476.20