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 $245.64   11.755%   $274.515   $274.52   $274.50   $274.50  
  Purchase #2 $51.96   8.515%   $56.3844   $56.38   $56.40   $56.40  
  Purchase #3 $93.62   8.273%   $101.3652   $101.37   $101.35   $101.40  
  Purchase #4 $23.73   7.177%   $25.4331   $25.43   $25.45   $25.40  
  Purchase #5 $72.46   7.297%   $77.7474   $77.75   $77.75   $77.70  
  Purchase #6 $194.63   5.315%   $204.9746   $204.97   $204.95   $205.00  
  Purchase #7 $176.88   3.212%   $182.5614   $182.56   $182.55   $182.60  
  7 purchase totals = $858.92       $922.9811   $922.98   $922.95   $923.00