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 $11.45   8.784%   $12.4558   $12.46   $12.45   $12.50  
  Purchase #2 $39.75   13.712%   $45.2005   $45.20   $45.20   $45.20  
  Purchase #3 $237.48   12.607%   $267.4191   $267.42   $267.40   $267.40  
  Purchase #4 $188.71   6.660%   $201.2781   $201.28   $201.30   $201.30  
  Purchase #5 $39.29   10.416%   $43.3824   $43.38   $43.40   $43.40  
  Purchase #6 $80.42   10.475%   $88.844   $88.84   $88.85   $88.80  
  Purchase #7 $133.72   3.592%   $138.5232   $138.52   $138.50   $138.50  
  7 purchase totals = $730.82       $797.1031   $797.10   $797.10   $797.10