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 $49.70   12.589%   $55.9567   $55.96   $55.95   $56.00  
  Purchase #2 $221.30   8.838%   $240.8585   $240.86   $240.85   $240.90  
  Purchase #3 $163.62   11.109%   $181.7965   $181.80   $181.80   $181.80  
  Purchase #4 $39.29   10.315%   $43.3428   $43.34   $43.35   $43.30  
  Purchase #5 $184.63   5.581%   $194.9342   $194.93   $194.95   $194.90  
  Purchase #6 $34.31   13.241%   $38.853   $38.85   $38.85   $38.90  
  Purchase #7 $53.68   4.713%   $56.2099   $56.21   $56.20   $56.20  
  7 purchase totals = $746.53       $811.9516   $811.95   $811.95   $812.00