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 $93.57   6.806%   $99.9384   $99.94   $99.95   $99.90  
  Purchase #2 $99.96   7.175%   $107.1321   $107.13   $107.15   $107.10  
  Purchase #3 $246.78   3.350%   $255.0471   $255.05   $255.05   $255.00  
  Purchase #4 $7.50   8.124%   $8.1093   $8.11   $8.10   $8.10  
  Purchase #5 $70.31   5.571%   $74.227   $74.23   $74.25   $74.20  
  Purchase #6 $42.98   5.594%   $45.3843   $45.38   $45.40   $45.40  
  Purchase #7 $61.93   9.670%   $67.9186   $67.92   $67.90   $67.90  
  7 purchase totals = $623.03       $657.7568   $657.76   $657.80   $657.60