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 $7.44   12.332%   $8.3575   $8.36   $8.35   $8.40  
  Purchase #2 $79.27   13.385%   $89.8803   $89.88   $89.90   $89.90  
  Purchase #3 $196.35   9.256%   $214.5242   $214.52   $214.50   $214.50  
  Purchase #4 $114.56   12.317%   $128.6704   $128.67   $128.65   $128.70  
  Purchase #5 $36.89   3.129%   $38.0443   $38.04   $38.05   $38.00  
  Purchase #6 $33.96   7.545%   $36.5223   $36.52   $36.50   $36.50  
  Purchase #7 $28.25   6.173%   $29.9939   $29.99   $30.00   $30.00  
  7 purchase totals = $496.72       $545.9929   $545.98   $545.95   $546.00