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 $119.59   6.316%   $127.1433   $127.14   $127.15   $127.10  
  Purchase #2 $8.35   4.657%   $8.7389   $8.74   $8.75   $8.70  
  Purchase #3 $144.79   3.148%   $149.348   $149.35   $149.35   $149.30  
  Purchase #4 $50.39   12.647%   $56.7628   $56.76   $56.75   $56.80  
  Purchase #5 $187.27   4.253%   $195.2346   $195.23   $195.25   $195.20  
  Purchase #6 $75.86   12.506%   $85.3471   $85.35   $85.35   $85.30  
  Purchase #7 $245.70   4.755%   $257.383   $257.38   $257.40   $257.40  
  7 purchase totals = $831.95       $879.9577   $879.95   $880.00   $879.80