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 $84.53   10.502%   $93.4073   $93.41   $93.40   $93.40  
  Purchase #2 $63.68   3.577%   $65.9578   $65.96   $65.95   $66.00  
  Purchase #3 $189.47   12.200%   $212.5853   $212.59   $212.60   $212.60  
  Purchase #4 $128.31   7.294%   $137.6689   $137.67   $137.65   $137.70  
  Purchase #5 $51.64   4.655%   $54.0438   $54.04   $54.05   $54.00  
  Purchase #6 $84.67   13.829%   $96.379   $96.38   $96.40   $96.40  
  Purchase #7 $139.41   3.822%   $144.7383   $144.74   $144.75   $144.70  
  7 purchase totals = $741.71       $804.7804   $804.79   $804.80   $804.80