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 $131.40   7.666%   $141.4731   $141.47   $141.45   $141.50  
  Purchase #2 $88.38   12.165%   $99.1314   $99.13   $99.15   $99.10  
  Purchase #3 $8.41   4.506%   $8.789   $8.79   $8.80   $8.80  
  Purchase #4 $209.30   9.482%   $229.1458   $229.15   $229.15   $229.10  
  Purchase #5 $62.73   9.638%   $68.7759   $68.78   $68.80   $68.80  
  Purchase #6 $74.38   10.396%   $82.1125   $82.11   $82.10   $82.10  
  Purchase #7 $15.60   13.365%   $17.6849   $17.68   $17.70   $17.70  
  7 purchase totals = $590.20       $647.1126   $647.11   $647.15   $647.10