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 $29.72   3.722%   $30.8262   $30.83   $30.85   $30.80  
  Purchase #2 $28.65   6.546%   $30.5254   $30.53   $30.55   $30.50  
  Purchase #3 $74.77   5.579%   $78.9414   $78.94   $78.95   $78.90  
  Purchase #4 $194.99   7.666%   $209.9379   $209.94   $209.95   $209.90  
  Purchase #5 $10.95   9.765%   $12.0193   $12.02   $12.00   $12.00  
  Purchase #6 $75.96   11.104%   $84.3946   $84.39   $84.40   $84.40  
  Purchase #7 $29.34   8.464%   $31.8233   $31.82   $31.80   $31.80  
  7 purchase totals = $444.38       $478.4681   $478.47   $478.50   $478.30