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 $227.98   12.468%   $256.4045   $256.40   $256.40   $256.40  
  Purchase #2 $177.74   13.515%   $201.7616   $201.76   $201.75   $201.80  
  Purchase #3 $141.51   13.593%   $160.7455   $160.75   $160.75   $160.70  
  Purchase #4 $55.71   11.291%   $62.0002   $62.00   $62.00   $62.00  
  Purchase #5 $22.90   4.563%   $23.9449   $23.94   $23.95   $23.90  
  Purchase #6 $208.84   4.557%   $218.3568   $218.36   $218.35   $218.40  
  Purchase #7 $223.93   6.317%   $238.0757   $238.08   $238.10   $238.10  
  7 purchase totals = $1,058.61       $1,161.2892   $1,161.29   $1,161.30   $1,161.30