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 $75.61   8.328%   $81.9068   $81.91   $81.90   $81.90  
  Purchase #2 $223.97   10.119%   $246.6335   $246.63   $246.65   $246.60  
  Purchase #3 $231.76   10.572%   $256.2617   $256.26   $256.25   $256.30  
  Purchase #4 $107.79   12.701%   $121.4804   $121.48   $121.50   $121.50  
  Purchase #5 $211.33   9.754%   $231.9431   $231.94   $231.95   $231.90  
  Purchase #6 $131.52   5.485%   $138.7339   $138.73   $138.75   $138.70  
  Purchase #7 $203.36   3.320%   $210.1116   $210.11   $210.10   $210.10  
  7 purchase totals = $1,185.34       $1,287.0710   $1,287.06   $1,287.10   $1,287.00