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 $16.64   11.677%   $18.5831   $18.58   $18.60   $18.60  
  Purchase #2 $80.92   11.523%   $90.2444   $90.24   $90.25   $90.20  
  Purchase #3 $35.90   9.669%   $39.3712   $39.37   $39.35   $39.40  
  Purchase #4 $36.59   7.345%   $39.2775   $39.28   $39.30   $39.30  
  Purchase #5 $27.73   9.361%   $30.3258   $30.33   $30.35   $30.30  
  Purchase #6 $197.48   3.442%   $204.2773   $204.28   $204.30   $204.30  
  Purchase #7 $141.51   9.560%   $155.0384   $155.04   $155.05   $155.00  
  7 purchase totals = $536.77       $577.1177   $577.12   $577.20   $577.10