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 $6.50   3.745%   $6.7434   $6.74   $6.75   $6.70  
  Purchase #2 $18.43   7.442%   $19.8016   $19.80   $19.80   $19.80  
  Purchase #3 $149.73   3.602%   $155.1233   $155.12   $155.10   $155.10  
  Purchase #4 $147.79   5.502%   $155.9214   $155.92   $155.90   $155.90  
  Purchase #5 $200.44   10.352%   $221.1895   $221.19   $221.20   $221.20  
  Purchase #6 $125.30   12.170%   $140.549   $140.55   $140.55   $140.50  
  Purchase #7 $38.80   13.137%   $43.8972   $43.90   $43.90   $43.90  
  7 purchase totals = $686.99       $743.2254   $743.22   $743.20   $743.10