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 $35.27   13.385%   $39.9909   $39.99   $40.00   $40.00  
  Purchase #2 $231.70   13.551%   $263.0977   $263.10   $263.10   $263.10  
  Purchase #3 $115.85   6.332%   $123.1856   $123.19   $123.20   $123.20  
  Purchase #4 $79.59   9.322%   $87.0094   $87.01   $87.00   $87.00  
  Purchase #5 $181.35   5.474%   $191.2771   $191.28   $191.30   $191.30  
  Purchase #6 $127.36   5.536%   $134.4106   $134.41   $134.40   $134.40  
  Purchase #7 $27.61   8.422%   $29.9353   $29.94   $29.95   $29.90  
  7 purchase totals = $798.73       $868.9066   $868.92   $868.95   $868.90