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 $239.75   7.376%   $257.434   $257.43   $257.45   $257.40  
  Purchase #2 $64.54   6.231%   $68.5615   $68.56   $68.55   $68.60  
  Purchase #3 $154.75   3.634%   $160.3736   $160.37   $160.35   $160.40  
  Purchase #4 $179.90   4.417%   $187.8462   $187.85   $187.85   $187.80  
  Purchase #5 $151.78   10.577%   $167.8338   $167.83   $167.85   $167.80  
  Purchase #6 $23.33   12.721%   $26.2978   $26.30   $26.30   $26.30  
  Purchase #7 $170.83   6.695%   $182.2671   $182.27   $182.25   $182.30  
  7 purchase totals = $984.88       $1,050.6140   $1,050.61   $1,050.60   $1,050.60