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 $246.32   10.562%   $272.3363   $272.34   $272.35   $272.30  
  Purchase #2 $217.71   11.746%   $243.2822   $243.28   $243.30   $243.30  
  Purchase #3 $11.42   5.346%   $12.0305   $12.03   $12.05   $12.00  
  Purchase #4 $247.60   9.703%   $271.6246   $271.62   $271.60   $271.60  
  Purchase #5 $62.42   10.814%   $69.1701   $69.17   $69.15   $69.20  
  Purchase #6 $92.99   11.433%   $103.6215   $103.62   $103.60   $103.60  
  Purchase #7 $45.38   6.112%   $48.1536   $48.15   $48.15   $48.20  
  7 purchase totals = $923.84       $1,020.2188   $1,020.21   $1,020.20   $1,020.20