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 $104.27   12.580%   $117.3872   $117.39   $117.40   $117.40  
  Purchase #2 $13.57   10.305%   $14.9684   $14.97   $14.95   $15.00  
  Purchase #3 $128.78   6.578%   $137.2511   $137.25   $137.25   $137.30  
  Purchase #4 $12.80   4.827%   $13.4179   $13.42   $13.40   $13.40  
  Purchase #5 $166.72   7.791%   $179.7092   $179.71   $179.70   $179.70  
  Purchase #6 $84.38   11.263%   $93.8837   $93.88   $93.90   $93.90  
  Purchase #7 $127.57   11.659%   $142.4434   $142.44   $142.45   $142.40  
  7 purchase totals = $638.09       $699.0609   $699.06   $699.05   $699.10