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 $121.65   4.246%   $126.8153   $126.82   $126.80   $126.80  
  Purchase #2 $76.74   10.113%   $84.5007   $84.50   $84.50   $84.50  
  Purchase #3 $30.59   3.854%   $31.7689   $31.77   $31.75   $31.80  
  Purchase #4 $232.48   3.716%   $241.119   $241.12   $241.10   $241.10  
  Purchase #5 $185.80   13.747%   $211.3419   $211.34   $211.35   $211.30  
  Purchase #6 $87.27   13.798%   $99.3115   $99.31   $99.30   $99.30  
  Purchase #7 $187.26   10.417%   $206.7669   $206.77   $206.75   $206.80  
  7 purchase totals = $921.79       $1,001.6242   $1,001.63   $1,001.55   $1,001.60