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 $29.90   6.614%   $31.8776   $31.88   $31.90   $31.90  
  Purchase #2 $39.85   5.312%   $41.9668   $41.97   $41.95   $42.00  
  Purchase #3 $18.54   7.410%   $19.9138   $19.91   $19.90   $19.90  
  Purchase #4 $46.89   3.539%   $48.5494   $48.55   $48.55   $48.50  
  Purchase #5 $229.76   12.200%   $257.7907   $257.79   $257.80   $257.80  
  Purchase #6 $143.42   11.471%   $159.8717   $159.87   $159.85   $159.90  
  Purchase #7 $91.60   3.612%   $94.9086   $94.91   $94.90   $94.90  
  7 purchase totals = $599.96       $654.8786   $654.88   $654.85   $654.90