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 $37.83   5.666%   $39.9734   $39.97   $39.95   $40.00  
  Purchase #2 $35.91   9.338%   $39.2633   $39.26   $39.25   $39.30  
  Purchase #3 $221.25   3.442%   $228.8654   $228.87   $228.85   $228.90  
  Purchase #4 $110.33   3.158%   $113.8142   $113.81   $113.80   $113.80  
  Purchase #5 $168.64   12.416%   $189.5783   $189.58   $189.60   $189.60  
  Purchase #6 $123.93   12.457%   $139.368   $139.37   $139.35   $139.40  
  Purchase #7 $24.31   3.528%   $25.1677   $25.17   $25.15   $25.20  
  7 purchase totals = $722.20       $776.0303   $776.03   $775.95   $776.20