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 $115.35   4.771%   $120.8533   $120.85   $120.85   $120.90  
  Purchase #2 $8.85   11.831%   $9.897   $9.90   $9.90   $9.90  
  Purchase #3 $25.70   12.793%   $28.9878   $28.99   $29.00   $29.00  
  Purchase #4 $70.56   6.808%   $75.3637   $75.36   $75.35   $75.40  
  Purchase #5 $75.95   4.268%   $79.1915   $79.19   $79.20   $79.20  
  Purchase #6 $119.47   10.673%   $132.221   $132.22   $132.20   $132.20  
  Purchase #7 $83.54   12.398%   $93.8973   $93.90   $93.90   $93.90  
  7 purchase totals = $499.42       $540.4116   $540.41   $540.40   $540.50