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 $18.58   12.848%   $20.9672   $20.97   $20.95   $21.00  
  Purchase #2 $235.31   3.189%   $242.814   $242.81   $242.80   $242.80  
  Purchase #3 $15.84   11.374%   $17.6416   $17.64   $17.65   $17.60  
  Purchase #4 $99.50   13.410%   $112.843   $112.84   $112.85   $112.80  
  Purchase #5 $113.71   12.745%   $128.2023   $128.20   $128.20   $128.20  
  Purchase #6 $33.72   5.460%   $35.5611   $35.56   $35.55   $35.60  
  Purchase #7 $181.53   4.323%   $189.3775   $189.38   $189.40   $189.40  
  7 purchase totals = $698.19       $747.4067   $747.40   $747.40   $747.40