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 $131.80   7.843%   $142.1371   $142.14   $142.15   $142.10  
  Purchase #2 $20.42   6.800%   $21.8086   $21.81   $21.80   $21.80  
  Purchase #3 $204.54   8.310%   $221.5373   $221.54   $221.55   $221.50  
  Purchase #4 $132.35   7.370%   $142.1042   $142.10   $142.10   $142.10  
  Purchase #5 $144.41   12.697%   $162.7457   $162.75   $162.75   $162.70  
  Purchase #6 $83.92   9.523%   $91.9117   $91.91   $91.90   $91.90  
  Purchase #7 $99.67   9.351%   $108.9901   $108.99   $109.00   $109.00  
  7 purchase totals = $817.11       $891.2347   $891.24   $891.25   $891.10