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 $222.93   11.842%   $249.3294   $249.33   $249.35   $249.30  
  Purchase #2 $38.54   13.740%   $43.8354   $43.84   $43.85   $43.80  
  Purchase #3 $141.60   13.233%   $160.3379   $160.34   $160.35   $160.30  
  Purchase #4 $48.29   8.237%   $52.2676   $52.27   $52.25   $52.30  
  Purchase #5 $239.81   8.429%   $260.0236   $260.02   $260.00   $260.00  
  Purchase #6 $31.29   5.321%   $32.9549   $32.95   $32.95   $33.00  
  Purchase #7 $237.80   7.647%   $255.9846   $255.98   $256.00   $256.00  
  7 purchase totals = $960.26       $1,054.7334   $1,054.73   $1,054.75   $1,054.70