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 $35.44   12.817%   $39.9823   $39.98   $40.00   $40.00  
  Purchase #2 $128.69   5.343%   $135.5659   $135.57   $135.55   $135.60  
  Purchase #3 $134.33   9.555%   $147.1652   $147.17   $147.15   $147.20  
  Purchase #4 $139.73   6.759%   $149.1744   $149.17   $149.15   $149.20  
  Purchase #5 $146.60   4.702%   $153.4931   $153.49   $153.50   $153.50  
  Purchase #6 $217.78   9.853%   $239.2379   $239.24   $239.25   $239.20  
  Purchase #7 $203.97   10.457%   $225.2991   $225.30   $225.30   $225.30  
  7 purchase totals = $1,006.54       $1,089.9179   $1,089.92   $1,089.90   $1,090.00