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 $79.80   6.656%   $85.1115   $85.11   $85.10   $85.10  
  Purchase #2 $17.92   12.685%   $20.1932   $20.19   $20.20   $20.20  
  Purchase #3 $7.29   7.188%   $7.814   $7.81   $7.80   $7.80  
  Purchase #4 $193.41   10.736%   $214.1745   $214.17   $214.15   $214.20  
  Purchase #5 $25.91   10.147%   $28.5391   $28.54   $28.55   $28.50  
  Purchase #6 $151.26   7.523%   $162.6393   $162.64   $162.65   $162.60  
  Purchase #7 $90.40   12.117%   $101.3538   $101.35   $101.35   $101.40  
  7 purchase totals = $565.99       $619.8254   $619.81   $619.80   $619.80