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 $154.65   7.541%   $166.3122   $166.31   $166.30   $166.30  
  Purchase #2 $132.63   12.811%   $149.6212   $149.62   $149.60   $149.60  
  Purchase #3 $33.62   4.177%   $35.0243   $35.02   $35.00   $35.00  
  Purchase #4 $189.58   9.395%   $207.391   $207.39   $207.40   $207.40  
  Purchase #5 $196.46   5.864%   $207.9804   $207.98   $208.00   $208.00  
  Purchase #6 $25.27   11.635%   $28.2102   $28.21   $28.20   $28.20  
  Purchase #7 $55.94   12.292%   $62.8161   $62.82   $62.80   $62.80  
  7 purchase totals = $788.15       $857.3554   $857.35   $857.30   $857.30