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 $20.27   3.455%   $20.9703   $20.97   $20.95   $21.00  
  Purchase #2 $39.41   5.666%   $41.643   $41.64   $41.65   $41.60  
  Purchase #3 $206.92   11.122%   $229.9336   $229.93   $229.95   $229.90  
  Purchase #4 $165.56   9.271%   $180.9091   $180.91   $180.90   $180.90  
  Purchase #5 $106.68   10.616%   $118.0051   $118.01   $118.00   $118.00  
  Purchase #6 $59.60   4.177%   $62.0895   $62.09   $62.10   $62.10  
  Purchase #7 $234.66   12.630%   $264.2976   $264.30   $264.30   $264.30  
  7 purchase totals = $833.10       $917.8482   $917.85   $917.85   $917.80