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 $19.92   5.416%   $20.9989   $21.00   $21.00   $21.00  
  Purchase #2 $212.50   10.668%   $235.1695   $235.17   $235.15   $235.20  
  Purchase #3 $184.54   3.653%   $191.2812   $191.28   $191.30   $191.30  
  Purchase #4 $239.60   3.177%   $247.2121   $247.21   $247.20   $247.20  
  Purchase #5 $240.57   12.412%   $270.4295   $270.43   $270.45   $270.40  
  Purchase #6 $142.77   7.697%   $153.759   $153.76   $153.75   $153.80  
  Purchase #7 $94.92   12.138%   $106.4414   $106.44   $106.45   $106.40  
  7 purchase totals = $1,134.82       $1,225.2916   $1,225.29   $1,225.30   $1,225.30