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 $134.65   12.392%   $151.3358   $151.34   $151.35   $151.30  
  Purchase #2 $140.64   10.518%   $155.4325   $155.43   $155.45   $155.40  
  Purchase #3 $114.44   9.129%   $124.8872   $124.89   $124.90   $124.90  
  Purchase #4 $119.43   12.355%   $134.1856   $134.19   $134.20   $134.20  
  Purchase #5 $160.94   12.674%   $181.3375   $181.34   $181.35   $181.30  
  Purchase #6 $136.87   13.296%   $155.0682   $155.07   $155.05   $155.10  
  Purchase #7 $59.34   9.277%   $64.845   $64.85   $64.85   $64.80  
  7 purchase totals = $866.31       $967.0918   $967.11   $967.15   $967.00