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 $212.75   8.711%   $231.2827   $231.28   $231.30   $231.30  
  Purchase #2 $156.35   13.396%   $177.2946   $177.29   $177.30   $177.30  
  Purchase #3 $103.55   13.500%   $117.5293   $117.53   $117.55   $117.50  
  Purchase #4 $226.71   7.184%   $242.9968   $243.00   $243.00   $243.00  
  Purchase #5 $66.77   11.577%   $74.5   $74.50   $74.50   $74.50  
  Purchase #6 $143.92   4.655%   $150.6195   $150.62   $150.60   $150.60  
  Purchase #7 $80.83   4.830%   $84.7341   $84.73   $84.75   $84.70  
  7 purchase totals = $990.88       $1,078.9570   $1,078.95   $1,079.00   $1,078.90