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 $18.52   12.683%   $20.8689   $20.87   $20.85   $20.90  
  Purchase #2 $104.78   13.435%   $118.8572   $118.86   $118.85   $118.90  
  Purchase #3 $235.95   10.577%   $260.9064   $260.91   $260.90   $260.90  
  Purchase #4 $14.77   9.601%   $16.1881   $16.19   $16.20   $16.20  
  Purchase #5 $73.48   7.535%   $79.0167   $79.02   $79.00   $79.00  
  Purchase #6 $27.48   3.207%   $28.3613   $28.36   $28.35   $28.40  
  Purchase #7 $239.82   11.577%   $267.584   $267.58   $267.60   $267.60  
  7 purchase totals = $714.80       $791.7826   $791.79   $791.75   $791.90