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 $223.31   11.415%   $248.8008   $248.80   $248.80   $248.80  
  Purchase #2 $215.67   9.597%   $236.3678   $236.37   $236.35   $236.40  
  Purchase #3 $195.90   4.237%   $204.2003   $204.20   $204.20   $204.20  
  Purchase #4 $211.56   4.589%   $221.2685   $221.27   $221.25   $221.30  
  Purchase #5 $67.64   5.668%   $71.4738   $71.47   $71.45   $71.50  
  Purchase #6 $239.97   8.375%   $260.0675   $260.07   $260.05   $260.10  
  Purchase #7 $87.95   8.864%   $95.7459   $95.75   $95.75   $95.70  
  7 purchase totals = $1,242.00       $1,337.9246   $1,337.93   $1,337.85   $1,338.00