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 $17.72   5.441%   $18.6841   $18.68   $18.70   $18.70  
  Purchase #2 $108.78   10.249%   $119.9289   $119.93   $119.95   $119.90  
  Purchase #3 $104.82   10.790%   $116.1301   $116.13   $116.15   $116.10  
  Purchase #4 $10.96   10.446%   $12.1049   $12.10   $12.10   $12.10  
  Purchase #5 $37.79   10.783%   $41.8649   $41.86   $41.85   $41.90  
  Purchase #6 $120.35   7.390%   $129.2439   $129.24   $129.25   $129.20  
  Purchase #7 $63.81   9.464%   $69.849   $69.85   $69.85   $69.80  
  7 purchase totals = $464.23       $507.8058   $507.79   $507.85   $507.70