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 $129.96   4.298%   $135.5457   $135.55   $135.55   $135.50  
  Purchase #2 $159.75   3.171%   $164.8157   $164.82   $164.80   $164.80  
  Purchase #3 $119.67   8.243%   $129.5344   $129.53   $129.55   $129.50  
  Purchase #4 $91.75   11.276%   $102.0957   $102.10   $102.10   $102.10  
  Purchase #5 $225.33   3.761%   $233.8047   $233.80   $233.80   $233.80  
  Purchase #6 $30.92   10.207%   $34.076   $34.08   $34.10   $34.10  
  Purchase #7 $130.70   6.851%   $139.6543   $139.65   $139.65   $139.70  
  7 purchase totals = $888.08       $939.5265   $939.53   $939.55   $939.50