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 $99.71   4.267%   $103.9646   $103.96   $103.95   $104.00  
  Purchase #2 $173.70   3.783%   $180.2711   $180.27   $180.25   $180.30  
  Purchase #3 $36.93   8.102%   $39.9221   $39.92   $39.90   $39.90  
  Purchase #4 $194.56   9.169%   $212.3992   $212.40   $212.40   $212.40  
  Purchase #5 $53.37   9.412%   $58.3932   $58.39   $58.40   $58.40  
  Purchase #6 $120.84   7.229%   $129.5755   $129.58   $129.60   $129.60  
  Purchase #7 $101.79   4.123%   $105.9868   $105.99   $106.00   $106.00  
  7 purchase totals = $780.90       $830.5125   $830.51   $830.50   $830.60