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 $79.28   12.350%   $89.0711   $89.07   $89.05   $89.10  
  Purchase #2 $9.75   6.402%   $10.3742   $10.37   $10.35   $10.40  
  Purchase #3 $136.60   9.687%   $149.8324   $149.83   $149.85   $149.80  
  Purchase #4 $144.28   3.241%   $148.9561   $148.96   $148.95   $149.00  
  Purchase #5 $129.68   11.614%   $144.741   $144.74   $144.75   $144.70  
  Purchase #6 $186.76   4.800%   $195.7245   $195.72   $195.70   $195.70  
  Purchase #7 $149.26   6.293%   $158.6529   $158.65   $158.65   $158.70  
  7 purchase totals = $835.61       $897.3522   $897.34   $897.30   $897.40