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 $5.49   5.293%   $5.7806   $5.78   $5.80   $5.80  
  Purchase #2 $129.58   9.259%   $141.5778   $141.58   $141.60   $141.60  
  Purchase #3 $122.52   7.743%   $132.0067   $132.01   $132.00   $132.00  
  Purchase #4 $117.38   10.593%   $129.8141   $129.81   $129.80   $129.80  
  Purchase #5 $29.90   6.566%   $31.8632   $31.86   $31.85   $31.90  
  Purchase #6 $110.98   11.645%   $123.9036   $123.90   $123.90   $123.90  
  Purchase #7 $76.84   9.829%   $84.3926   $84.39   $84.40   $84.40  
  7 purchase totals = $592.69       $649.3386   $649.33   $649.35   $649.40