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 $232.45   3.345%   $240.2255   $240.23   $240.25   $240.20  
  Purchase #2 $219.56   5.777%   $232.244   $232.24   $232.25   $232.20  
  Purchase #3 $201.40   12.731%   $227.0402   $227.04   $227.05   $227.00  
  Purchase #4 $184.70   3.634%   $191.412   $191.41   $191.40   $191.40  
  Purchase #5 $151.72   11.442%   $169.0798   $169.08   $169.10   $169.10  
  Purchase #6 $29.43   7.314%   $31.5825   $31.58   $31.60   $31.60  
  Purchase #7 $177.54   11.459%   $197.8843   $197.88   $197.90   $197.90  
  7 purchase totals = $1,196.80       $1,289.4683   $1,289.46   $1,289.55   $1,289.40