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 $26.37   9.504%   $28.8762   $28.88   $28.90   $28.90  
  Purchase #2 $91.34   12.783%   $103.016   $103.02   $103.00   $103.00  
  Purchase #3 $91.60   6.391%   $97.4542   $97.45   $97.45   $97.50  
  Purchase #4 $110.47   6.120%   $117.2308   $117.23   $117.25   $117.20  
  Purchase #5 $62.57   13.106%   $70.7704   $70.77   $70.75   $70.80  
  Purchase #6 $169.59   3.612%   $175.7156   $175.72   $175.70   $175.70  
  Purchase #7 $66.81   10.232%   $73.646   $73.65   $73.65   $73.60  
  7 purchase totals = $618.75       $666.7092   $666.72   $666.70   $666.70