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 $182.73   11.411%   $203.5813   $203.58   $203.60   $203.60  
  Purchase #2 $72.43   10.244%   $79.8497   $79.85   $79.85   $79.80  
  Purchase #3 $131.31   4.466%   $137.1743   $137.17   $137.15   $137.20  
  Purchase #4 $138.89   9.367%   $151.8998   $151.90   $151.90   $151.90  
  Purchase #5 $227.94   10.359%   $251.5523   $251.55   $251.55   $251.60  
  Purchase #6 $91.40   13.786%   $104.0004   $104.00   $104.00   $104.00  
  Purchase #7 $58.59   11.416%   $65.2786   $65.28   $65.30   $65.30  
  7 purchase totals = $903.29       $993.3364   $993.33   $993.35   $993.40