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 $48.52   12.723%   $54.6932   $54.69   $54.70   $54.70  
  Purchase #2 $126.73   4.517%   $132.4544   $132.45   $132.45   $132.50  
  Purchase #3 $172.68   13.245%   $195.5515   $195.55   $195.55   $195.60  
  Purchase #4 $249.47   9.487%   $273.1372   $273.14   $273.15   $273.10  
  Purchase #5 $167.37   8.394%   $181.419   $181.42   $181.40   $181.40  
  Purchase #6 $236.82   11.102%   $263.1118   $263.11   $263.10   $263.10  
  Purchase #7 $150.71   12.518%   $169.5759   $169.58   $169.60   $169.60  
  7 purchase totals = $1,152.30       $1,269.9430   $1,269.94   $1,269.95   $1,270.00