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 $5.42   7.292%   $5.8152   $5.82   $5.80   $5.80  
  Purchase #2 $149.69   6.869%   $159.9722   $159.97   $159.95   $160.00  
  Purchase #3 $91.75   13.667%   $104.2895   $104.29   $104.30   $104.30  
  Purchase #4 $7.79   11.828%   $8.7114   $8.71   $8.70   $8.70  
  Purchase #5 $75.88   7.516%   $81.5831   $81.58   $81.60   $81.60  
  Purchase #6 $118.55   12.731%   $133.6426   $133.64   $133.65   $133.60  
  Purchase #7 $76.63   13.346%   $86.857   $86.86   $86.85   $86.90  
  7 purchase totals = $525.71       $580.8710   $580.87   $580.85   $580.90