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 $128.32   11.338%   $142.8689   $142.87   $142.85   $142.90  
  Purchase #2 $8.31   5.460%   $8.7637   $8.76   $8.75   $8.80  
  Purchase #3 $128.74   3.567%   $133.3322   $133.33   $133.35   $133.30  
  Purchase #4 $38.62   6.557%   $41.1523   $41.15   $41.15   $41.20  
  Purchase #5 $6.47   9.496%   $7.0844   $7.08   $7.10   $7.10  
  Purchase #6 $38.87   13.353%   $44.0603   $44.06   $44.05   $44.10  
  Purchase #7 $157.93   9.422%   $172.8102   $172.81   $172.80   $172.80  
  7 purchase totals = $507.26       $550.0720   $550.06   $550.05   $550.20