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 $86.51   4.128%   $90.0811   $90.08   $90.10   $90.10  
  Purchase #2 $13.30   7.634%   $14.3153   $14.32   $14.30   $14.30  
  Purchase #3 $133.99   10.276%   $147.7588   $147.76   $147.75   $147.80  
  Purchase #4 $15.71   6.495%   $16.7304   $16.73   $16.75   $16.70  
  Purchase #5 $132.91   6.588%   $141.6661   $141.67   $141.65   $141.70  
  Purchase #6 $145.95   3.567%   $151.156   $151.16   $151.15   $151.20  
  Purchase #7 $11.37   4.166%   $11.8437   $11.84   $11.85   $11.80  
  7 purchase totals = $539.74       $573.5514   $573.56   $573.55   $573.60