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 $81.67   10.583%   $90.3131   $90.31   $90.30   $90.30  
  Purchase #2 $143.59   8.647%   $156.0062   $156.01   $156.00   $156.00  
  Purchase #3 $147.94   6.606%   $157.7129   $157.71   $157.70   $157.70  
  Purchase #4 $29.89   4.757%   $31.3119   $31.31   $31.30   $31.30  
  Purchase #5 $13.88   10.180%   $15.293   $15.29   $15.30   $15.30  
  Purchase #6 $19.64   12.736%   $22.1414   $22.14   $22.15   $22.10  
  Purchase #7 $63.60   4.348%   $66.3653   $66.37   $66.35   $66.40  
  7 purchase totals = $500.21       $539.1438   $539.14   $539.10   $539.10