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 $238.77   6.274%   $253.7504   $253.75   $253.75   $253.80  
  Purchase #2 $129.67   6.401%   $137.9702   $137.97   $137.95   $138.00  
  Purchase #3 $247.99   6.440%   $263.9606   $263.96   $263.95   $264.00  
  Purchase #4 $41.36   13.361%   $46.8861   $46.89   $46.90   $46.90  
  Purchase #5 $8.91   6.422%   $9.4822   $9.48   $9.50   $9.50  
  Purchase #6 $41.27   8.146%   $44.6319   $44.63   $44.65   $44.60  
  Purchase #7 $170.51   6.530%   $181.6443   $181.64   $181.65   $181.60  
  7 purchase totals = $878.48       $938.3257   $938.32   $938.35   $938.40