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 $246.74   3.486%   $255.3414   $255.34   $255.35   $255.30  
  Purchase #2 $170.28   13.561%   $193.3717   $193.37   $193.35   $193.40  
  Purchase #3 $95.48   4.854%   $100.1146   $100.11   $100.10   $100.10  
  Purchase #4 $84.65   6.715%   $90.3342   $90.33   $90.35   $90.30  
  Purchase #5 $203.66   7.485%   $218.904   $218.90   $218.90   $218.90  
  Purchase #6 $11.75   4.751%   $12.3082   $12.31   $12.30   $12.30  
  Purchase #7 $101.76   4.824%   $106.6689   $106.67   $106.65   $106.70  
  7 purchase totals = $914.32       $977.0430   $977.03   $977.00   $977.00