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 $45.71   10.572%   $50.5425   $50.54   $50.55   $50.50  
  Purchase #2 $210.89   9.494%   $230.9119   $230.91   $230.90   $230.90  
  Purchase #3 $121.80   7.314%   $130.7085   $130.71   $130.70   $130.70  
  Purchase #4 $197.79   9.566%   $216.7106   $216.71   $216.70   $216.70  
  Purchase #5 $216.85   4.813%   $227.287   $227.29   $227.30   $227.30  
  Purchase #6 $210.69   5.323%   $221.905   $221.91   $221.90   $221.90  
  Purchase #7 $161.60   10.290%   $178.2286   $178.23   $178.25   $178.20  
  7 purchase totals = $1,165.33       $1,256.2941   $1,256.30   $1,256.30   $1,256.20