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 $120.96   11.617%   $135.0119   $135.01   $135.00   $135.00  
  Purchase #2 $176.55   8.624%   $191.7757   $191.78   $191.80   $191.80  
  Purchase #3 $134.77   9.789%   $147.9626   $147.96   $147.95   $148.00  
  Purchase #4 $109.62   9.235%   $119.7434   $119.74   $119.75   $119.70  
  Purchase #5 $67.53   3.697%   $70.0266   $70.03   $70.05   $70.00  
  Purchase #6 $243.30   4.225%   $253.5794   $253.58   $253.60   $253.60  
  Purchase #7 $198.31   7.606%   $213.3935   $213.39   $213.40   $213.40  
  7 purchase totals = $1,051.04       $1,131.4931   $1,131.49   $1,131.55   $1,131.50